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Glossary of Standard Grade Physics Terms

By Chris McGinlay, ©Sandwick Junior High School 2000, 2001,2002.

This is a glossary of the terms used in Standard Grade Physics. Let the entire document load or get 'Transfer Interrupted' errors! Most diagrams or figures will be kept in separate linked documents. There are approximately 275 definitions at present.

. Not all terms are inter-related. Still to be error checked.Webmaster. Please notify me of any errors or suggestions.



Acceleration is the rate of increase of velocity. Acceleration tells you how much faster or slower a moving object gets every second. A negative acceleration is called a deceleration. The SI Unit of acceleration is the metre per second per second (ms-2)
Acceleration due to Gravity
This is the acceleration caused by the gravitational pull of a planet. On Earth, the strength of the acceleration due to gravity is roughly 10 ms-2. The acceleration due to gravity is often given the symbol 'g'. See also weight and mass
Accommodation relates to the ability of the eye to change its focal length and so focus on objects at a range of distances. To focus on a close object, the eye lens will be fat (having a short focal length). To focus on a far object, the lens will be pulled into a thin shape having a long focal length. Failure of accommodation gives rise to long-sightedness and short-sightedness.
The activity of a sample of radioactive material is the number of disintegrations per second taking place in that particular sample. The SI unit of activity is the becquerel (Bq).
Aerodynamic objects have minimal air resistance. This means that they can move at greater speed through the air. Remember that as objects go faster the aerodynamic drag increases. Therefore, an aerodynamic object will be able to travel faster than a non-aerodynamic one!
Alpha (a) Radiation
a (alpha) radiation consists of slow moving helium nuclei. These nuclei are released during the disintegration of an unstable nucleus. a radiation will be stopped by a few centimetres of air, or a sheet of paper. It is a strongly ionising radiation, due in part to its relatively large mass - it is about 8000 times as massive as the electrons in b radiation! The helium nuclei are positively charged.
Alternating Current (a.c.)
Electrical current continually changes direction in an a.c. power supply (as opposed to direct current (d.c.))

In the UK, mains electricity is a.c., with a frequency of 50 Hz and an r.m.s voltage of 230 V.

This is a device for generating a.c. electricity. It consists of rotor coils (with a d.c. supply) and stator coils which produce the a.c. electricity.
The ampere is the SI unit of electrical current. At a simple level it can be defined as the number of coulombs of electrical charge flowing per second. The full definition is beyond the scope of the S-grade course
An amplifier is a component of an electronic system, such as a radio or TV which makes all the input signals louder. Ideally, an amplifier will increase the amplitude of all input signals by the same factor. Amplifiers require a power supply which can be battery or mains. The voltage gain or the power gain can be used to describe how much amplification is taking place.
Amplitude (Diagram)
This is the height of a wave, measured vertically from the centre line to a crest or a trough. The SI unit is the metre, although in some contexts amplitudes may be given in volts.
Amplitude Modulation (Diagram)
Amplitude modulation refers to the changing of the amplitude of a radio-frequency or microwave frequency carrier wave. The other method of modulation is called frequency modulation.
Analogue signals should be compared with digital signals. Analogue signals are continuous. As an example, a microphone is an analogue input device. This is because it can produce a range of voltage levels.
AND Gate
An AND gate is a logical device. It usually takes two inputs (although more are possible) and produces only one output. The logic state of the output depends on the logic state of the inputs. This is shown in the truth table below. A and B are the inputs and Z is the output state.

From this it is easy to see that the output is on only if both inputs are both on.

Angle of Incidence (Diagram 1) (Diagram 2 )
The angle between the normal line and a ray or wave incident to a surface.
Angle of Reflection (Diagram)
The angle between the normal line and a ray or wave reflected from a surface
A multicoil rotating assembly, used in commercial motors instead of the single rotating coil of the model motor built in class. A segmented commutator is used to connect each of the armature windings to the power supply in turn, thus providing smoother rotation of the motor.
An asteriod is a lump of rock which orbits around the sun. (The biggest asteroids are about 100km across, although many are 1km or less across. Asteroids orbit the Sun in the asteroid belt, between the orbits of Mars and Jupiter.
An atom is the smallest possible particle of any chemical element. For example, an atom of carbon is the smallest possible unit of carbon. All of the atoms of an element are identical to each other, but different from the atoms of any other element. For example, all carbon atoms are essentially identical to each other, but carbon atoms and oxygen atoms are different from each other. The size of an atom is approximately 10-10m.

The reason why atoms of oxygen are different from carbon (or any other element) is down to the structure within the atom itself. Atoms are made up from protons, neutrons and electrons. Atoms of different elements have different numbers of protons. Atoms of the same element have the same number of protons. For example, atoms of carbon always have six protons. Atoms of oxygen always have eight protons.

The protons and neutrons are found in the nucleus of the atom. The tiny electrons 'orbit' the nucleus. The nucleus is very small, approximate diameter is 10-14m.

See also: Ion.

This link has more information on atoms

Angle of Refraction (Diagram)
The angle between the normal line and a ray or wave refracted at a surface
Average Speed
The average speed of a vehicle is usually calculated over the whole distance and time of the journey. For example a train may cover a distance of 240 km in a time of 3 hours. The average speed can be worked out as follows:

In kilometres per hour: speed = 240 km / 3 hr = 80 km/hr

In metres per second: speed = 240 000 m / 10 800 s = 22.2 m/s

Remember that average speed is different from instantaneous speed.

Audio Wave
A sound wave, but usually in the context of modulation and radio transmission


Background Radiation
Background radiation is the radiation which occurs naturally in the rocks, air and water around us.
Battery (Diagram)
A battery is a number (or battery) of cells, connected in series with the positive terminal of one cell connected to the negative of the next. Batteries provide a d.c.(direct current) source of electrical energy.
b (beta) radiation
b (beta) radiation consists of fast moving electrons. These electrons are produced as a result of transitions that take place in unstable nuclei. b radiation can be stopped by a few millimetres of aluminium. It is an ionising radiation, not as strongly ionising as a (alpha) radiation
Big-bang Theory
The 'big-bang theory' of cosmology tries to explain the origin of the universe as an explosive event, where space, time and matter came into being. Evidence for the big-bang includes redshift measurements (and microwave background radiation studies of the universe - look up CMBR on the web if you are interested.
A system of counting in twos instead of in tens (counting in tens is called decimal). Here is a comparison of the decimal and binary counting system:

You should notice that each column (or place) in decimal is ten times the value of the previous one, whereas in binary, each column is only twice the value of the previous one. Also in decimal, you can have any digit from 0 up to 9 in any of the columns, whereas in binary, you can have either a zero or a one in any of the columns. In the example given, you can see that 12 in decimal is the same as 1100 in binary.

Black Hole
A collapsed star from which no light can escape, due to the intense gravitational field.
Block Diagram
In electronics, a block diagram provides a simple way of analysing how a system works. Block diagrams don't show details of how components work, but simply show how information or signals should travel through the system. They are usually drawn using rectangles and arrows.


Cancer is a growth of out-of-control cells. Localised cancer growths are called tumours. a, b, g or X-ray radiation can be used to diagnose and treat cancer.
Capacitor (Diagram)
A capacitor is a device which can store electrical charge. Capacitors are often used to introduce a time delay in electronic circuits. The SI unit of capacitance is the farad.
Carbon Dating
See radiometric dating.
Carrier Wave
Carrier waves are used to carry information from transmitter to receiver. The carrier wave will be radio or microwave frequency. There are two methods of using the carrier wave to carry information - amplitude modulation and frequency modulation. In radio transmissions for example, the carrier is of a much higher frequency than the audio wave which is to be transmitted. The higher the frequency of the carrier, the higher the quality of the transmitted signal.
Cell (Diagram)
In electricity, a cell is a source of d.c. electricity. A cell can be made using two different metals inserted into a salt solution, for example. The metals react with the salt solution at different rates and in so doing, generate a small electrical current. Many cells can be combined to produce a battery of cells.

In biology, cells are the basic building blocks of all living things. Plant and animal cells alike have a tiny nucleus which controls the behaviour of the cell, a cell membrane which controls the entry and exit of substances. The inside of the cell is filled with a liquid called cytoplasm. These cells can be damaged or destroyed by ionising radiation such as a, b, or g. The amount of cell damage caused by radiation depends on:

See also: Dose Equivalent.

There are two types of electrical charge. One type is called 'positive', the other is called 'negative'. Like charges repel each other, whilst unlike charges attract each other. The amount of charge affects how strong this attraction or repulsion actually is.

The SI unit of charge is the coulomb (C)

See also the references for the electron, the proton and the neutron.

In electrical circuits, charge is related to current by the equation Q = I t

Chemical Potential Energy
Chemical energy is a form of potential energy because it is stored energy. Energy stored in food, torch batteries and explosives like dynamite is all chemical energy. Chemical energy is released in a chemical reaction (like combustion of explosives or respiration in plant and animal cells).
Clock Pulse Generator
A simple circuit using a capacitor, resistor and transistor. The circuit generates 'clock pulses' - simply a regular sequence of logic LO and logic HI. The capacitor charges and discharges through the resistor. As the capacitor charges to a certain level, the inverter's input goes HI and its output goes LO. This this causes the capacitor to start discharging. When its voltage gets low enough, the inverter output goes back to HI and the cycle starts again.

By increasing the resistance or increasing the capacitance, the pulses can be generated more slowly.

Combined Heat and Power
Most power stations end up trying to turn heat energy into electrical energy. This can never be done with 100% efficiency (due to the second law of thermodynamics). Most of the energy produced (by combustion for example) will be lost as waste heat. A combined heat and power plant uses the waste heat to heat nearby homes and factories. This is why half of Lerwick was dug up around 2000-2001 - for the installation of pipes for the CHP plant.
(Image 1) SP
Hale Bopp over Hoswick, 
Shetland, 1997

A comet is a small body orbiting our Sun, usually at great distance from Earth and with a highly elliptical orbit. They are composed of 'ices' and dust. As a comet approaches the inner solar system, the heat of the Sun vapourises volatile material from the surface and pushes the material directly away from the Sun. This gives rise to the 'ion tail', usually a ghostly blue colour and quite straight, it always points directly away from the Sun and has nothing to do with the direction of motion of the comet. A second tail exists, usually white and curved. This is dust which falls off the comet and is left in track of its orbit. Here is a photo of comet Hale-Bopp, taken from Hoswick, Shetland in 1998. Scientists recently deliberately crashed a space probe onto the surface of a comet in order to learn more.

Comets and their dust tails give rise to regular meteor showers.

A material that allows electricity to flow easily through it. Copper, for example is a good conductor. Good conductors have a very low resistance. See also insulators and semi-conductors
Conduction (Heat)
This refers to the ability of heat energy to travel through a material. For example, copper is a good conductor of heat as it will heat up quickly if left on a radiator. Wood, for example does not conduct heat so well. Conduction is not possible in a vacuum. See also convection and radiation.
Consumer Unit
Commonly known as the fusebox, the consumer unit is the part of the meter board which separates the incoming electrical supply into the various lighting and ring main circuits. Each of these circuits is protected by either a fuse or miniature circuit breaker (MCB)
Continuous Spectrum
The light from a tungsten filament lamp has a range of colours present within it. Each colour corresponds to a wavelength of visible (EM) radiation. In a continuous spectrum like this, there are no missing wavelengths. However, the light from the Sun (or from any star) has certain wavelengths missing. The missing wavelengths can be used to identify the 'fingerprint' of chemical elements in stars' atmospheres, or in the intervening dust and gas clouds between us and a particular star. In class, we use a direct vision spectroscope to observe the missing colours of sunlight.
Control Rods
These are used in nuclear reactors. They control the rate of nuclear reactions by absorbing the neutrons which cause the reactions to happen. By lowering the rods into the reactor, the rate of reaction slows down.
This is a way of moving heat energy around. In a fluid (i.e. a liquid or gas) heat can travel by convection. For example, hot air is less dense than cold air, so the hot air rises up, carrying its heat energy with it. Convection is not possible in a vacuum. See also conduction and radiation.
Core (Soft Iron)
The presence of an iron core in a solenoid will produce a stronger magnetic field than the solenoid would manage on its own. The iron core thus magnifies the effect of the solenoid's magnetic field.
The coulomb is the SI Unit of electrical charge. It can be defined as the amount of charge which flows in 1 second due to a current of 1 ampere.
Crest (Diagram)
This refers to waves. The crest of a wave (also called the peak) is simply the part of the wave with the greatest amplitude.
Critical Angle
The critical angle is associated with refraction of light. This angle is the smallest angle of incidence at which total internal reflection (TIR) occurs.
Electrical current is the rate of flow of electrical charge round a circuit. It is measured in amperes and should be measured in series with an ammeter. See also: Ohm's Law
Curved Reflector (Diagram)
Curved reflectors, with a parabolic shape, are used to produce parallel beams, or to collect light, radio waves or similar, over the area of the reflector and focus this energy onto a detector. This makes the received signal at the detector much greater than if no reflector was used.
A complete cycle of a wave is equivalent to one complete wavelength of that wave. The time taken to produce one complete cycle is called the period of the wave. The number of cycles per second is called the frequency of the wave


Direct Current (d.c.)
Electrical current only flows in one direction from a d.c. power supply (as opposed to alternating current (a.c.))
Deceleration basically means slowing down - reducing speed. A negative answer for an acceleration would actually be a deceleration. Like acceleration, deceleration is also measured in ms-2.
Decibel (dB) Scale
The decibel scale is used to measure the volume (intensity or loudness) of sound. The abbreviation for this unit is dB. For the S-grade physics course, a sound level of 80dB is considered to be the danger level - i.e. the level at which hearing damage starts to occur.
The decoder section of a radio essentially removes the carrier wave from the modulated signal, leaving only the audio wave which is then amplified. In TVs there are two separate decoders - one for the picture and one for the sound.
Deflection Plates
Deflection plates are used to direct the electron beam to all parts of the cathode ray tube of an oscilloscope. There are two sets of plates, one set for controlling horizontal movement of the beam and the other set for vertical movement. Each set of plates controls the beam by having a certain voltage applied across the plates.
Diffraction is a wave phenomenon - i.e. all types of wave can diffract. Diffraction means the bending of wave (change in direction) and it occurs whenever a wave passes near to an obstacle. Longer wavelength waves diffract through a greater angle than shorter wavelength waves. Do not confuse diffraction with refraction.
Digital can refer to electronic signals or to components of an electronic system. Digital signals have only two voltage levels, described as 'HI' and 'LO', or 'OFF' and 'ON'. In a real component, 'HI' might be represented by +5V d.c. and 'LO' by 0V d.c. A digital component is one which works with only digital signals. Here are some examples of digital components: LED (output), photodiode (input), switch (input). Contrast digital signals with analogue signals.
Diode (Diagram)
A diode is a semi-conductor device. Diodes will allow current to pass through them in only one direction.
Discharge Lamps
See under Gas Discharge Lamps
Double Insulation (Diagram)
An electrical appliance which is double insulated does not have an earth wire fitted. The appliance is designed in such a way that the electrical parts can never come into contact with the outer casing of the device. Common double insulated appliances are hair dryers, radios and cassette players.

A wet double insulated appliance is exceptionally dangerous - water is a good conductor of electricity and will easily reach the live electrical components within the case. Any human user touching the casing will then receive an electric shock. It is important to note that the human conductivity is increased when hands are wet, allowing a greater current to flow through the casualty.

For this reason, do not operate a mains radio, hairdryer or double insulated appliance in any wet area - especially a bathroom

Do not touch any person who has been electrocuted until you are sure that the electricity supply has been shut off.

Dose Equivalent
This is the biological risk due to exposure to radiation. The SI unit of dose equivalent is the sievert (Sv). The damage caused to cells depends on three major factors (see under cells).
Drag is a type of friction force usually associated with movement through a fluid like air or water. Drag forces generally increase at high speeds.
A device (a tranducer) which converts kinetic energy into electrical energy. Dynamos are often used to produce a.c. electricity on a push-bike. Unlike a full-blown generator, the rotor coil is replaced with rotating permanent magnets. These magnets rotate near to the stator coil, (wrapped around a soft iron core) and cause a small current to flow (via electromagnetic induction).


Earth (planet)
Planet Earth has the third most distant orbit from our star the Sun. To the best of our knowledge it is the only place in the entire universe where we know that life exists (well, some people have made this claim at least) - although this may change as we explore Jupiter's moon Europa. It is the only place in the Solar System where large quantities of liquid water exist (oceans).
Earth (wire)
If fitted, the earth core of a flex connects the casing of the electrical appliance to ground via the earth terminal of the plug. Its insulation is coloured green and yellow. In the event of an appliance malfunction resulting in the casing becoming live, the earth wire provides a route to ground for the current. This will help prevent electric shock. If the earth wire is not present and the casing is live, any person touching the casing will receive a shock as the current flows to earth through them. If the earth wire is intact, its low resistance will result in a very high current flowing through live. This should cause the fuse in the plug to blow and remove any fire hazard.

Note that some devices are double insulated and have no earth. These devices must not be used near water or steam

Do not touch any person who has been electrocuted until you are sure that the electricity supply has been shut off.

No machine can be 100% efficient. Efficiency is defined as 'Useful Energy Out' / 'Total Energy In'. It is a fraction, sometimes written as a percentage. For example, if an electric motor in a winch consumes 2000J of energy per second, and does 420J of useful work per second, then the efficiency of the winch is, eff = 420/2000 = 0.21 = 21%. Note that efficiency has no units. Note that since 2000J per second is the same as 2000W power, you can do the efficiency calculations with power too.
Electromagnet (Diagram)
An electromagnet is a solenoid with an iron core inserted into it. If a current flows in the coil, a magnetic field is generated. All the randomly oriented domains of the iron core then align in the presence of the field of the solenoid. Thus, the core greatly enhances the strength of the electromagnet.
Electromagnetic Induction
If a conductor is moved within a magnetic field, then a current will be induced (caused) in the conductor. If the magnetic field is reversed or if the direction of movement is reversed, then the direction of the current flow will also reverse.

This is a very similar effect to the movement of a current carrying wire in a magnetic field: a non-magnetisable copper wire placed in a magnetic field can be made to move if a current is passed through it. If either the direction of the magnetic field or the direction of the current is reversed, then the movement will be in the opposite direction.

Electromagnetic Spectrum
See also under spectrum. This includes includes Gamma, X-rays, Ultraviolet, Visible, Infra-red, Microwave and Radio - in order of increasing wavelength.
Electrons are sub-atomic particles. They carry a negative electrical charge (qe=-1.602 x 10-19 C).
They have a mass of me = 9.110 x 10-31 kg.

Beams of electrons are used in TV sets where they are called cathode rays. See also b (beta) radiation.

Electronic System
An electronic system can simply be considered as consisting of three parts: input, process and output. Input devices include microphones and LDRs, thermistors and switches. Process sections can be made from one or more transistors, perhaps built into logic gates or computer chips. The output section will consist of one or more output devices, for example, a buzzer, LED, lamp, CRT.
It is difficult to define in a precise way what energy actually is. For the Standard grade, it is enough to think of energy as the ability to do mechanical work - to lift up weights for example. There are many different types of energy including: The SI unit of energy is the joule (J). Energy can not be created or destroyed, but can be changed from one form to another. It is an abstract concept, but makes many problems in science easier to solve.
The various parts of the eye are shown in a diagram (not included yet). Light entering the eye passes through (in order) the cornea, aqueous humour, pupil, lens and vitreous humour before striking the retina. Electrical signals from the retina are sent to the brain via the optic nerve. The pupil is actually a hole in the iris. The lens is held in place by the ciliary muscles and suspensory ligaments.

Common eye conditions are long(presbyopia)- and short(myopia)- sightedness.

Eyepiece Lens
The lens on a telescope (or microscope) which the observer places his or her eye next to. The eyepiece's sole funciton is to magnify the image. image


The farad (F) is the SI Unit of capacitance. Since the farad is a massive unit, most capicitances are given in micro-farads. The definition of the farad is beyond the scope of the standard grade course, but it is based on measurements of voltage and charge.
See under long-sighted.
Fibrescope are sometimes called endoscopes. Fibrescopes use optical fibres to allow doctors to view inside a patient without cutting them open. There are two separate fibre bundles - one for taking light down into the patient, called the lightguide. The other is for forming an image and is called the image guide. The image guide must be a coherent fibre bundle.
Filament Lamp
These lamps rely on an incandescent metal filament to produce light. The interior of the lamp is filled with an inert gas in order to stop the filament from corroding. An electrical current, passed through the filament makes it extremely hot. When sufficiently hot, the filament itself will emit light radiation. Tungsten metal is chosen for the filament due to its high melting point of about 3956 celsius.

Note that filament lamps produce both light and unwanted heat. As such they are less efficient than fluorescent tubes

Fission Process
This usually refers to nuclear fission, where an atomic nucleus splits apart into smaller pieces. Fission should not be confused with fusion. When a heavy, unstable nucleus undergoes fission, energy is released as heat. In nuclear power stations, uranium is often used as the fissile material, where a nuclear chain reaction is controlled to release energy.
The flex allows electrical current to flow into domestic appliances from the mains socket. The flex should be correctly wired to a plug.
Fluorescent Lamps
These lamps produce light by passing electrical current through a mercury vapour at low pressure. The electrical current excites ("energises") the electrons of the mercury atoms. These electrons quickly give up their energy as light. The light which is produced is invisible ultra-violet and would be useless. To get round this problem, the inside of the tube is coated with fluorescent paint, which converts U.V. light to visible light.

Note that fluorescent lamps are more efficient than filament lamps as they waste less energy as heat

Focal Length
The focal length of lens is measured in metres. It is the distance from the centre of the lens to the point at which rays passing through the lens meet (i.e. focus). It is related to lens power.
Forces can cause an object to speed up, slow down, change direction or change shape (if they are unbalanced). Forces are either push forces or pull forces. The SI unit of force is the newton.
Fossil Fuel
Fossil fuels are coal, oil and gas. They are non-renewable resources. Fossil fuels come from ancient remains of plants and animals, compressed and heated over many millenia. The combustion of fossil fuels is now (nearly) universally accepted as causing global warming. They are well known to produce acid rain. Most of the world's energy is still produced from fossil fuel. Here in Shetland, we paid the price in the form of the Braer tanker disaster.
The frequency of a regular event is the number of times the event occurs in a given time. e.g. The Sun rises with a frequency of once per day.

The frequency of a wave is the number of complete wavelengths which pass any point in one second. The SI unit of frequency is the hertz (Hz) (equivalent to 'per second' or s-1).

The concept of wave period is closely related to frequency.

See also: cycle.

Frequency Modulation
Frequency modulation refers to the changing of the frequency of a radio-frequency or microwave frequency carrier wave. The other method of modulation is called amplitude modulation.
Friction is a force which tends to make moving objects slow down. Friction also prevents objects from slipping over one another - so for example, friction helps keep your backside on a seat and it allows people to stand upright without their feet slipping out from under them. Friction due to fluid flow is called drag.
A fridge is an example of a heat pump. It extracts heat energy from inside the fridge cabinet and releases it into the room. The basic principle relies on the latent heat of vapourisation of a coolant liqid. The coolent is evaporated within the pipes inside the cabinet - the heat required for this is taken from the cabinet itself, with the result that the cabinet becomes cooler.
A fuse, where fitted, is designed to protect the flex (and not the appliance) from current overload and the associated risk of fire. The fuse rating should always be less than that of the flex which it is protecting. This will make sure that it blows before the flex melts. If the fuse in the plug blows, it is likely to be because of a fault in the appliance which is drawing too much current.

The fuse (and switch) should always be connected to the live wire in a plug.



A grouping of about approximately 100 billion stars, held together by their own gravity. We live in the Milky Way galaxy, visible on any clear, moonless night away from Lerwick street-lights. Within a galaxy, there are many different types of star, some older, younger, brighter, dimmer, or more/less massive than the Sun. Galaxies do not all look the same - astronomer's classify them according to their appearance. Ask your teacher to show you the Andromeda galaxy - easily visible on a clear, dark night as a large misty patch. Andromeda is one of the nearest galaxies to our galaxy (the Milky Way), at a distance of two million light-years.
Gamma (g) Radiation
g (gamma) radiation is high frequency electromagnetic radiation. It is a part of the electromagnetic spectrum. Gamma radiation carries no mass, no charge, travels at the speed of light. (It can be thought of as being made of particles called photons or as being a wave). It can be stopped by several centimetres of lead or a few metres of concrete. Gamma radiation is an ionising radiation, but is not as strongly ionising as a radiation. It is released during the disintegration of an unstable nucleus.

For the purposes of treating cancer, medical gamma rays from cobalt-60 are used.

Gamma Camera
This is a device used to detect the gamma radiation from a patient during an examination using a radioactive tracer. Since gamma radiation passes straight through most materials, it is not possible to build a camera with glass lenses or mirrors. Instead an array of holes drilled in a thick layer of lead is placed between the actual gamma detectors and the patient. This allows an image to be built up. Scintillation counters are used to detect the radiation after it has passed through the holes in the commutator.
Gas Discharge Lamps
These lamps produce light by passing electrical current through a gas at low pressure. The electrical current excites ("energises") the electrons of the atoms in the gas. These electrons quickly give up their energy as light. Sodium is often used (in street lights) or an inert gas for colourful advertising lights.

Note that discharge lamps are more efficient than filament lamps as they waste less energy as heat

'Geostationary' refers to a satellite of planet Earth. Such a satellite will appear to hover above the surface of the Earth, since it is placed in an orbit with a period of 24 hours.
Geothermal Energy
This is a renewable energy resource, unlike fossil fuels. In some parts of the world, hot subsurface rock heats water which reaches the surface and can be used for heating or electrical power generation. Geothermal Resources Counil
A machine which uses motion to produce electrical energy. Sometimes called an alternator. A generator is like a dynamo, but with the permanent rotor magnets replaced with (d.c) rotor coils (electromagnets). The stator coils have an a.c. induced within them.
Gravitational Field
The Earth generates a gravitational field which attracts all other masses. It is a called a force field - the word field means that the force changes as you move away from the surface of the Earth (in fact the force becomes steadily less as you move away from the surface of the Earth). It is the combined mass of all particles on Earth which cause the Earth's gravitational field. The strength of the field is measured in Nkg-1.
Gravitational Potential Energy
If a mass is lifted up then its potential energy is given as follows:

Ep = mgh

Ep is the potential energy

m is the mass in kilograms

g is the strength of the gravitational field (10ms-2 on Earth)

h is the height through which the object is lifted

Geiger-Muller tube
This is a device used to detect the presence of radiation. It relies on ionisation to produce tiny bursts of current which can be counted by a ratemeter. A popular exam question, and worth checking your class notes.
Gold Grain
Radioactive gold grains are used in the treatment of cancer. Gold (which is unreactive) doped with radioactive (gamma emitting) nuclei is implanted in a patient, near or in a tumour. The radiation will destroy or damage cells near the grain. The radioactive material chosen will have a short half-life so that the grain's activity level will drop to background levels after several days.


The half-life of a radioactive material is the time taken for its activity level to drop by a half. Half-life is measured using any unit of time (seconds, hours etc). For example, if the activity of a sample is 20MBq at 12:00 pm, falling to 10 MBq at 2:00 pm, then the half-life is 2 hours. At 4:00 pm the activity will be 5 MBq, etc. There is a dramatic range in the half-life from element to element. For example the half-life of 113Cd (cadmium) is 9 x 1015 years (nine million billion years), whilst the half-life of 213At (astatine) is only 0.11 millionths of a second!
Heat Energy
Heat is a form of energy and is not to be confused with temperature. There is no such thing as 'cold energy'. At the molecular level, heat energy is transferred from particle to particle by vibration (this is called conduction). A particle vibrating violently can be said to have a lot of heat energy. As well as by conduction, heat can also be transferred by convection or radiation.
The operation of a bar heater is very similar indeed to a filament lamp. The difference is in the thickness of the 'filament' or element of the heater. Curiously, heaters operate at a lower temperature than lamps, but produce far more heat - as infra-red radiation. (Remember that temperature and heat are very different concepts). Note that some energy is wasted as (red) light.
Heat Pump
This is a device which can move heat energy from a colder object to a hotter one, but requiring an external energy source to do it. A fridge is an example of a heat pump. Due to the second law of thermodynamics, a heat pump can never be 100% efficient.
Hyperopia or hypermetropia (hyperopic or hypermetropic eye)
See under long-sighted.


A chemical element is inert if it does not react with anything. Examples are helium, neon, argon, krypton, xenon and radon. Inert gases can be used in filament lamps to protect the filament from destruction due corrosion.
Inertia is a property of matter. It describes how difficult it is to either get something moving or to stop it when it is moving. So an elephant has a lot more inertia than a paper clip. The SI unit for inertia is the kilogram. It's important to realise that, e.g. when in a spacecraft in orbit, the effects of gravity can be ignored, but inertia cannot - an astronaut can still be crushed by a stray satellite! See also: Mass.
Input Device
The first part of an electronic system, an input device typically processes requests from a human user. For example in a CD player, the 'ON' button and volume controls (potentiometer) are all input devices. Other input devices include switches, thermistors, LDRs, photodiodes.
Infrared radiation is invisible. It is a form of electromagnetic radiation, with a wavelength just longer than that of visible red light. Infrared light is use in telecommunications (via optical fibres). It is used in medicine to diagnose illness (thermograms) and accelerate healing in physiotherapy. Loosely speaking, infrared radiation can be thought of as 'heat rays'.
This means an event which occurs over a very short (infinitesimal) period of time. It usually refers to speed, i.e. instantaneous speed as opposed to average speed.
Instantaneous Speed
The speed at any particular stage or time during a journey. Instantaneous speed can be calculated by measuring short distances and time intervals. For example a train coming into a station may move forward by 0.2 m in a time of 0.5 s, giving an instantaneous speed of 0.4 m/s. Compare this with the average speed of the train over a whole journey.
A material which does not usually conduct electricity. Air, for example is an insulator. Insulators have a very high (ideally infinite) resistance. Given a suitably high voltage, insulators can be forced to conduct - take for example lightning during a thunderstorm. See also conductors and semi-conductors
Insulation (Heat)
An insulator in this sense is a material which does not conduct heat well. Ceramics, stone and plastics are good insulators against heat conduction. Metals are poor insulators as they will quickly heat up or cool down.
Inverter (NOT gate)
A processing device within an electronic system, the NOT gate is a single input logic gate which 'inverts' its input. A logic 'HI' input will become a logic 'LO' output and visa-versa. Here is the truth table:
An ion is an electrically charged atom. In a neutral atom, the number of protons in the nucleus is balanced by the number of electrons around the nucleus. If an electron is knocked away from the nucleus (perhaps as a result of radiation), then the atom will be left with a net positive charge. The process of creating ions is called ionisation.


The joule (J) is the SI Unit of energy and of (mechanical) work. One joule is the same as one newton-metre (1 J = 1 Nm).
Planet Jupiter has the fifth most distant orbit from our star the Sun. It is the largest planet of our solar system, with a diameter which is 11 times that of the Earth. A so-called 'gas giant', most of the planet is made of swirling clouds of toxic gas, with wind speeds up to 450 mph. (For comparison, typical wild winter storms in Shetland which reach well over 100mph, would be very tame).


The kelvin is the SI Unit of absolute temperature. 273.15 K is the same as 0 Celsius and 373.15 K is the same as 100 Celsius. Its formal definition is beyond the scope of the S-grade course.
The kilogram is the SI unit of mass. It is defined as the mass of a particular cylinder of platinum which is kept at the International Bureau of Weights and Measures at Sevres, near Paris.
Kilowatt-hour (kWh)
This is a unit of energy, usually used in electrical situations. Electricity companies bill their customers on the number of kWh used. One kWh means that energy has been used at a rate of 1000 W (Js-1) for one hour. Using this, the kWh can be related to the joule as follows:

E = P t

1 kWh = 1 000 Js-1 x 3 600 s

1 kWh = 3 600 000 J = 3.6 MJ

kWh is a unit of electrical energy and NOT power!

Kinetic Energy
Kinetic energy is the energy of motion. It is given by the formula:

Ek=0.5 * mv2

where, Ek is the kinetic energy in joules

m is the mass of the object in kilograms

v is the velocity of the object in metres per second.



A device intended to produce light energy. Modern lamps convert electrical energy to light energy via one of two main mechanisms - the filament lamp or the gas discharge lamp. Note that filament lamps are less efficient than discharge lamps (i.e. fluorescent lamps) as they produce more waste heat energy.
The word laser was originally an abbreviation for Light Amplification by the Stimulated Emission of Radiation. Lasers have countless applications, many in medicine and communications. Lasers produce light which is monochromatic, coherent and tightly focussed. Lasers are used in the technique of photocoagulation (used to treat retinae).
Latent Heat
Latent heat is heat energy used or released during change of phase.

When matter changes phase, energy must be given out or taken in. Change of phase takes place at constant temperature. For example, if a bunsen is used to bring 100ml of water to the boil, it is easy to understand that the heat energy from the bunsen increases the heat energy of the water - because the water gets hotter. During boiling however, the temperature of the water does not increase. The energy from the bunsen no longer causes an increase in temperature, but is now carried away in the steam. The energy is used to break the bonds between the water molecules instead of increasing the temperature. (Remember: heat and temperature are different concepts!)

Liquid Crystal Display (LCD)
This is an output device. They run on almost no power since they don't produce their own light, but work by reflecting room or daylight.
Light Dependent Resistor (LDR) (Diagram)
LDRs are designed so that their resistance depends on the intensity(brightness) of light falling on them.

When in low intensity illumination (dark!), the resistance of an LDR is high.

When in bright illumination, the resistance of an LDR is low.

Light Emitting Diode (Diagram)
LEDs are simply diodes which emit light when connected correctly to a power supply.
The light-year (ly) is a unit of distance which is useful in astronomy. It is the distance which light (or any other form of electromagnetic radiation) travels in one year. Note that real astronomers do not use light-years, but use a unit called the parsec instead.

It is easy to work out how far a light-year is as follows:

distance = speed x time

1 ly = 3 x 10 8 x 365x24x60x60 metres

1 ly = 9.46 x 10 15 metres

To appreciate the scale of this unit, recall that the distance from the Earth to the Sun is 1.5 x 10 8 metres. This is about 8 light-minutes. The distance to the next-nearest star (Proxima Centauri) is about 4 ly!
Line Spectrum
Plural: Line Spectra. A spectrum of light with only certain wavelenghts or colours present. (See also continuous spectrum). If you use a direct vision spectroscope to view light from a (yellow) sodium vapour street-lamp, you will see that only a few wavelengths are present. This is an emission line spectrum and is unique to sodium. Sodium can also absorb these wavelengths too, so if a continuous spectrum of white light were to be shone through sodium vapour and then observed through a spectroscope, certain colours or wavelengths would be missing. This is called an absorption spectrum.

These facts can be used to identify which elements are present in stars' atmospheres, by matching lines in their spectra with the spectra of elements tested on Earth.

Logic Diagram
A simplified diagram of an electronic system, using rectangles to represent input and output devices along with the usual logic gate symbols for the AND, OR and NOT gate etc. Inputs and outputs are given letters (A,B,C,.. for inputs; Z for output) to help layout a truth table.
Logic Gate
For standard grade, we study AND, OR and NOT gates. Other common gates are the NAND, NOR and XOR (or EOR). Logic gates allow an electronic system to make decisions based on a number of inputs. For example an OR gate can be used to illuminate a warning light if engine temperature is too high or if it is over-revved.
Logic State
In a digital system, there are two logical states. One state is 'ON', 'HI' or 'logic 1' (all mean the same). The other is 'OFF', 'LO', 'logic 0' (again all mean the same).
This refers to a problem with the accommodation ability of an eye. Basically, the eye can focus on far away objects with no problem, but can't properly focus on nearby objects. There can be two causes of this. The first occurs in older people, where the aging eye lens becomes less flexible and so cannot adopt the highly curved shape needed to focus at short range. This condition is called presbyopia. The second cause is that retina is too near to the lens - so that the focal point falls behind the retina. This is called hypermetropia or hyperopia. Long-sightedness can be treated by wearing spectacles which are convex in shape and therefore help to focus the image properly by reducing the overall focal length of the glasses and eye.
A lubricant is a substance which reduces friction between two surfaces. Examples are oil, water, air, bananas, seaweed.


A simple permanent magnet like a bar magnet has two poles, called 'North' and 'South'. Like poles repel and unlike poles attract. A magnet sets up a magnetic field in three dimensions around itself. Field lines can be drawn to try to represent this magnetic field. The arrows on a field line point from North to South and show the direction a North pole would move. Field lines never cross. Where they are closest together, the field is strongest.
Planet Mars has the fourth most distant orbit from our star the Sun. It is slightly smaller than Earth and has a thinner atmosphere, made of carbon dioxide. Mars has carbon dioxide ice caps at its north and south poles.
Mass refers to the quantity of matter present in an object. The SI unit of mass is the kilogram. In actual fact there are two kinds of mass - inertial mass and gravitational mass. Gravitational mass describes how an object responds to (and creates) a gravitational field.
Mercury (metal)
Mercury is a metal which is liquid at room temperature. It is often used in liquid in glass thermometers. It is a toxic substance. Chemicool web-page for mercury.
Mercury (planet)
Planet Mercury has closest orbit to our star the Sun. It is much smaller than Earth and has no atmosphere. It is very hard to see Mercury from Earth, due to it being very close to the Sun.
Commonly called 'shooting stars', meteors are tiny specks of dust which the Earth sweeps into in its orbit. These dust grains enter the Earth's atmosphere at up to 70km per second - very fast! They cause the atoms in the atmosphere which they hit to glow with the heat produced as the meteor slows down. The meteor is destroyed by this process. Many meteors are particles which are left behind in the orbit of comets as they pass through the inner solar system.
This is a lump of material which manages to reach Earth's surface, having only partially burned up on the way down.
These are small lumps of material found in space. They range from less than 1mm across, up to tens of metres. Only if a meteroid enters Earth's atmosphere it is called meteor.
The metre is the SI unit of distance. It is defined as distance light travels in the fraction 1 / 299 792 458 of a second.
A microphone is an input device in an electronic system. It converts sound energy into electrical energy. It is therefore a transducer.
Microwaves, along with light, UV, x-rays and gamma radiation form part of the electromagnetic spectrum. The wavelength of microwaves is longer than that of infra-red radiation, but shorter than that of radio waves. Microwaves are used commonly in mobile phones and in microwave ovens.
Milky Way
This is the name of the galaxy we live in. There are about 100 billion stars in the Milky Way. The Milky Way is a spiral galaxy, made up of several arms. Our arm is imaginatively called the 'local arm' (by us at least). The Milky Way actually has a few smaller satellite galaxies, two of them are the Large and Small Magellanic Clouds, visible from the southern hemisphere.
Miniature Circuit Breaker (MCB)
Instead of using a fuse to protect an electrical flex, a miniature circuit breaker (MCB) can be used instead. MCBs are sometimes known as Earth Leakage Circuit Breakers (ELCBs) or Residual Current Devices (RCDs). The MCB has several advantages over fuses as follows:

Disadvantages of the MCB are as follows:

The Australian government's website has more information on RCDs, useful for background

In a nuclear fission reactor, the moderator slows fast neutrons down. Slower moving neutrons are more effective at keeping the chain reaction going by causing more uranium nuclei to break apart. Each fission event releases fast neutrons - the moderator is therefore required to slow them down.
Modulation means change. In physics, this is usually used to describe changes made to the properties of a radio-wave or microwave carrier wave in order to carry information. There are two types of modulation - amplitude modulation and frequency modulation.
Morse Code
Morse Code is a versatile and simple method of transmitting information. It uses any method of telecommunication (such as flashing lights, tap keys and wires or radio transmissions) to send information coded as sequences of dots and dashes (e.g. short and long pulses of voltage or flashes of light). This is its great advantage over any other means of information transmission: morse can be sent with extremely simple equipment. Its disadvantage is that it has a very slow rate of transmission and takes considerable practice to use properly. The codes are shown in Telecommunications Activity 3
Motor (Diagram)
Motors convert electrical energy into kinetic energy, along with some unwanted sound and heat energy.

A simple motor, which can run on d.c. electricity, has a rotor coil and permanent field magnets. As the rotor coil spins, the direction of the current flowing in the coil is reversed by means of a split-ring commutator. Carbon brushes allow the commutator to slide whilst maintaining an electrical connection. Carbon brushes are used as carbon is a good conductor and is hard-wearing.

In commercial motors, the permanent field magnets are replaced with electromagnets called field coils. This allows a more powerful magnetic field to be created than would be possible using permanent magnets. Also, the single rotor coil of the simple motor is replaced by a number of rotor coils (called a multi-coil or armature) and a segmented commutator replaces the split ring commutator. Each coil of the armature is connected to a pair of contacts in the segmented commutator. This allows for a smoother rotation of the motor. Note that in a commercial motor, because the magnetic field is produced by electromagnets, the motor will work equally well with a.c. or d.c. electricity. This is because a reverse current in the field coil will coincide with a reversal in the armature current. Therefore the armature will be forced round in the same direction.

To read more about how motors work, try "How Stuff Works"

Myopia (Myopic eye)
See under short-sighted.


National Grid
This is a network of power lines which connect together all the mainland UK power stations. Grid controllers can monitor the demand for electrical energy across the UK and allow more power to be diverted to areas with high demands.
See under short-sighted.
Planet Neptune usually has the eighth most distant orbit from our star the Sun. It is gas giant, although smaller than Jupiter or Saturn. It was discovered after observing abnormalities in Uranus' orbit. Calculations were made and Neptune was observed where the figures said it had to be. Neptune has three very faint rings (not visible from Earth). Sometimes Pluto is closer to the Sun than Neptune.
Neutrons are sub-atomic particles. They are uncharged
They have a mass of mn = 1.675 x 10-27 kg.
Neutron Star
The remnant of a dead star after it has exploded in a supernova. Neutron stars are extremely dense and have intense graviational fields, so strong that all atoms are crushed, forcing their nuclei together. Electrons and protons from the smashed atoms are crushed together to form neutrons. Neutron stars are a few kilometres across and have masses comparable to that of the Sun! (100 million tonnes per teaspoonful)
Sir Isaac Newton, 1642-1727. Newton tried farming before going on to develop his three Laws of Motion (see below). Newton also contributed to other branches of science and maths.
The newton (N) is the SI unit of force. An unbalanced force of 1 newton will cause a mass of 1 kg to accelerate at 1ms-2
Newton Balance
This is a spring balance (called a pundler in Shetland). It can be used to measure force, including weight forces. Remember all forces are measured in newtons!
Newton's First Law
If the forces acting on an object are all balanced, then the object will not change in its motion. It will not speed up, slow down, change direction or shape. IF it is not moving, then it will stay stationary.
Newton's Second Law
If forces are not balanced on an object then it will either speed up, slow down or change direction - this means it will have to accelerate. Remember that a change of direction is also an acceleration, even if the speed does not change. The formula for this law is:

F = ma

F is the force in newtons, m is the mass in kilograms and a is the acceleration in ms-2

Newton's Third Law
For any force in a situation, there will be an equal and opposite force. For example if you stand on some grass, then your weight force pulls you down and the grass pushes you up with the same force, but in the opposite direction. This means that the forces are balanced - so by Newton's first law - you don't move.
Non-Renewable Energy Sources
Fossil fuels are termed non-renewable sources because they will run out one day. See also renewable energy sources.
Normal Line (Diagram)
In optics and wave applications, the normal line is an imaginary line drawn at right angles to a surface. If the surface is curved, then the tangent to the surface must be drawn first, with the normal being drawn at right angles to the tangent.
NOT gate (inverter)
See under inverter
Nuclear Chain Reaction
In a nuclear fission power station (or a nuclear fission bomb) an unstable nucleus of uranium is hit by a neutron, causing it to break up, releasing a small quantity of energy plus several more neutrons. These new neutrons go on to hit more nuclei, causing them to break up, releasing even more energy and neutrons. This quickly builds up to a nuclear chain reaction. In a power station, control rods are used to regulate the rate of energy release. In a fission bomb, the chain reaction is allowed to proceed out of control with horrendous consequences.
Nuclear Energy
Nuclear energy is the energy contained within the nucleus of an atom. It can be released in nuclear fission or nuclear fusion. Much of the world's electricity is produced using nuclear fisson power stations. Fusion power remains elusive, although progress is slowly being made.


Objective Lens
In a refracting telescope the objective lens is the large lens which collects the dim light from objects in the night sky. The purpose of the objective is to collect as much light as possible. It does not magnify the image (see eyepiece). With a larger objective, the image will be brighter and fainter objects will become visible.
Optical Fibres
Optical fibres rely on the principle of total internal reflection for their operation. Optical fibres are made from extremely thin, flexible hairlike strands of glass. They allow light to be sent over great distances and round corners. This makes possible devices such as the medical endoscope (or fibrescope) and various high capacity data communications. In communications applications, fibre optics have several advantages over copper wires - they are lighter in weight, more difficult to eavesdrop and require far fewer booster stations.
Open Circuit
This is one of two types of circuit fault (the other being a short circuit). In an open circuit fault, there is a break in the circuit. This break will prevent any current from flowing. Open circuit faults register on an ohmmeter with an extremely high (ideally infinite) resistance.
Ohm (W)
The ohm is the SI unit of resistance. The resistance of an electrical component is one ohm (1 W) if, when a voltage of 1 V applied across it causes a current of 1 A to flow through it.
Ohm's Law
Georg S. Ohm discovered that for a conductor at constant temperature, the current which flows through the conductor is proportional to the voltage across it. The constant of proportionality is called the resistance of the conductor. The law is usually written as:

V = I R

OR Gate
An OR gate is a logical device. It usually takes two inputs (although more are possible) and produces only one output. The logic state of the output depends on the logic state of the inputs. This is shown in the truth table below. A and B are the inputs and Z is the output state.

From this it is easy to see that the output is on if at least one of the inputs is on.

The oscilloscope provides a visual way of investigating electrical signals. In standard grade, you need to understand about the voltage gain and timebase controls of the oscilloscope. The vertical scale shows the voltage of the signal - its scale can be modified by changing the Y-gain setting (in V/div). The voltage can then be deduced by calculating V = Y-gain x div, where 'div' is the number of divisions on the oscilloscope screen. The horizontal scale shows time and its control is called the timebase (or X-gain) (in ms/div). The period T of a wave-like signal can be calculated from T = timebase x div, where 'div' is the number of divisions taken to draw one full cycle.


Parallel Beam (Diagram)
A parallel beam is one which does not diverge or converge - its width stays the same as the wave (which could be light or microwaves for example) travels out from its source. Parallel beams are produced using curved (parabolic) reflector dishes (e.g. in car headlamps, satellite transmitter dishes, mobile phone mast relay dishes). Parallel light beams can also be produced using convex lenses (e.g. in binoculars or telescopes). Parallel beams can be used to send waves over much greater distances than would otherwise be possible. This is because the wave energy stays much closer together when reaching its destination. However, this would be no use for sending a signal out which was intended to cover a wide area.

In astronomy, the light from stars has come from such a great distance that the starlight does effectively form a parallel beam travelling down the tube of a telescope.

Parallel Circuit
In a parallel circuit, each component is connected directly across the terminals of the power supply. Therefore, the voltage across any component is the same as the voltage across the power supply terminals. The rules for current, voltage and resistance in a parallel circuit are as follows:

Vsupply = V1 = V2 = V3 = ...

Isupply = I1 + I2 + I3 + ...

1/Rtotal = 1/R1 + 1/R2 + 1/R3 + ...

Note that the calculation for total resistance in parallel circuits requires an extra step to get the final answer. Here is an example calculation for a 30 ohm and a 40 ohm resistor connected in parallel:

1/Rtotal = 1/R1 + 1/R2

1/Rtotal = 1/40 + 1/30

1/Rtotal = 3/120 + 4/120 (change to common denominator)

1/Rtotal = 7/120

Rtotal/1 = 120/7 (extra step)

Rtotal = 17.14 W (remember the unit)

Calculators with a reciprocal button can make these calculations easier. See also: Series circuits

Satellite period is the time taken for a satellite to make one complete orbit around its parent body (e.g the time taken for the Moon to orbit the Earth is called the Moon's period). Satellite period is usually measured in hours or days. Wave period is the time taken for one complete wave to pass a point. The SI unit of wave period is the second (s). A closely related concept is that of frequency. See also: cycle.

Peak (Diagram)
This refers to waves. The peak of a wave (also called the crest) is simply the part of the wave with the greatest amplitude.
Peak Voltage
This the maximum voltage reached by an a.c. source in each cycle. It is very similar to the peak or crest of a wave. Compare with the r.m.s voltage of an a.c. source.
Persistence of Vision
This is the effect which allows the human brain to be fooled into thinking that the 25 frames per second of TV is actually continuous motion. The retina retains each image for a fraction of a second and so provided that the changes between the frames are small enough, smooth motion will be seen. (The phosphor dots which make up the screen also continue to emit light for a short time after the electron beam has passed - this effect also helps to give the impression of a smooth motion)
Photographich film can be used to detect visible light, x-rays, or alpha, beta and gamma radiation. These radiations will blacken film due to a chemical reaction. Photographic film badges are used by medical and nuclear workers to monitor their level of exposure to radiation.
Our solar system has nine planets, including the Earth. Pluto's status as a planet is a subject of current debate, with many scientists pointing out that it is really just a big rock. The table below shows some data for the planets. A planet does not produce its own light but shines by reflected star-light (sunlight!). The word 'planet' literally means 'wanderer' due to the way the planets move across the night sky. From Earth, we can easily observe Mars, Venus, Jupiter and Saturn using just binoculars or the naked eye. With 20x60 binoculars, firmly mounted it is also easy to observe Jupiter's four Galilean moons - a treat well worth the effort- they will be visible as tiny pin-points of light all in a row (but you may not see all four at once). These moons move around pretty rapidly and if you sketch their positions, you should easily see them shift position after several hours.
NameDistance from Sun (approx)/AUPeriodDay/hrMass/ mass of EarthRadius / radius of Earth
Mercury0.488 days58.7 days0.0550.382
Venus0.7226 days243 days0.8150.949
Earth1.01 year23.9 hours1.0001.000
Mars1.51.9 years24.6 hours0.1070.533
Jupiter5.211.9 years9.9 hours31811.2
Saturn9.529.5 years10.7 hours95.29.45
Uranus19.284.023.9 hours14.64.1
Neptune30.1165.017.8 hours17.23.9
Pluto39.4248.06.4 days0.0020.24

Mercury, Venus, Earth and Mars are called 'inner planets'. Jupiter, Saturn, Uranus, Neptune and Pluto are 'outer planets'.

Plug (Diagram)
A plug is correctly wired as follows:

Green/Yellow insulated core to the earth terminal (top)

Brown insulated core to the live terminal. The live terminal can be identified by the fuse. The fuse and the switch should always be connected to live as it is the live which supplies electrical energy to the device. If the fuse and switch were connected to neutral, the appliance could appear to be off, yet still be connected to live.

Blue insulated core to the neutral terminal.

Planet Pluto usually has the furthest orbit from our star the Sun. It is the smallest planet, really just a large lump of rock. Its status as a planet has been challenged by some, due to its similarity to other similar sized objects. Currently the IAF still list Pluto has a planet. Due to its highly elliptical orbit, Pluto is nearer to the Sun than Neptune for part of its orbit.
Potential Divider (potentiometer)
A variable resistor, when used with all three of its terminals, it is called a potentiometer or potential divider. This arrangement is for controlling voltage. See also: Rheostat.
Potential Energy
Potential energy means stored energy. Some examples are chemical, elastic, nuclear and gravitational - all forms of potential energy. For Standard grade, you should know the formula for gravitational potential energy.
Power is the rate of transfer of energy. The SI unit of power is the watt (W). One watt is the same as one joule per second

The formula which relates power (P), energy (E) and time (t) is:

E = P x t

In electrical applications, there are other equations for power, which involve current, voltage and resistance. They are:

P = I V

P = I2R

P = V2 / R

Power Gain
Power gain has no units of its own. It expresses how many times more power the output signal from a process has compared to its input power. A closely related concept is that of voltage gain. Power Gain = Output Power / Input Power.
Power (of a lens)
Lens power describes the curvature of a lens. High power lenses have a short focal length and are highly curved. A positive lens power indicates a convex lens and negative power indicates a concave lens. Lens power, P, is measured in Dioptres (D) and is related to the focal length, f, in metres through the formula:

P = 1 / f

Presbyopia (presbyopic eye)
See under long-sighted.
Primary Colour (Diagram)
There are three primary colours (of light): red, green and blue. Any two of these colours can be mixed to give a secondary colour as follows:

Red + Green = Yellow

Red + Blue = Magenta (pink)

Green + Blue = Cyan (Turquoise)

If all three colours are mixed together, white light can be obtained.

Note that these are the primary and secondary colours of light. The primary and secondary colours of pigment (paint) are not the same.

Primary Mirror
In a reflecting telescope, the primary mirror collects the light from the objects under study in the night sky. Reflectors can be made much bigger than the objective lenses of a refracting telescope. They are also easier to make, since their is only one optical surface to prepare. The primary mirror itself is usually made from glass, with an aluminium coating on its forward face (unlike the mirror in your bedroom). The mirror is made as large as possible for the same reasons as for the objective lens of a refractor. See also eyepiece.
Principle of Reversibility
This applies to light rays passing through any system of mirrors or lenses. Basically, it means if a ray of light follows a path when going, say, from left to right, then another ray of light going from right to left will follow the same path in reverse.
Protons are sub-atomic particles. They carry a positive electrical charge (qp = +1.602 x 10-19 C). They have a mass of mp = 1.673 x 10-27 kg.
Pumped Storage
At times of low electricity demand, typically at night, the National Grid has overcapacity. Since it is difficult and expensive to shut down power stations, this extra energy would have to be wasted. However, by using it to pump water back uphill into reservoirs, a small percentage of this energy can be stored as gravitational potential energy of the water. The process is inefficient, but dumping the excess power would be 0% efficient.


Quasar means 'quasi-stellar radar source'. These are objects which look like stars but are not. (They are only visible in big telescopes, because of their great distance from us. A typical quasar differs most noticeably from a star because it gives off the same intensity of light as would one million million suns. This is quite impressive, but even more impressive is the fact that quasars spew out much more radiation at radio wavelengths. Quasars are amongst the most distant objects observed from Earth.


Radar Gun
A hand-held device used by police to measure the speed of passing cars.
The word 'radiation' can refer to electromagnetic radiation (including gamma radiation) or to alpha and beta particles released from radioactive decay. Radiation can cause ionisation of neutral atoms, by knocking away electrons. This can damage or kill cells. In this way, radiation can cause cancer and be used to treat cancer.
Radiation (heat)
Heat radiation is a part of the electromagnetic spectrum, conventionally called 'infra-red'. Heat can be transferred from a hot object to a cold one by radiation, convection or conduction. Transfer by radiation is possible in a vacuum (or in presence of matter).
A radio is an electronic system which allows a listener to hear programmes sent from a transmitter. The main components of a radio (in order) are: the aerial, tuner, decoder, amplifier and loudspeaker.
A material is said to be radioactive if it emits radiation. Radioactive materials have unstable nuclei which have a tendency to decay. It is when a decay occurs that radiation is released from the nucleus. Important related concepts are the activity and half-life of a source.
Radioactive Decay
This refers to the disintegration of an unstable nucleus. The number of disintegrations per second taking place in the sample is called the activity of the sample. Radiation is given out when a disintegration takes place.

Considering each nucleus on its own, radioactive decay is a random process. There is no way of knowing when a particular nucleus will decay - it may decay within the next millionth of a second, or it may not decay for another billion years! This isn't a measurement problem - the information just doesn't exist.

However, when you take a very large number of nuclei, the number of nuclei decaying (i.e. the activity) becomes predictable. Once a nucleus has decayed and given out radiation, the remains of the nucleus will still be in whatever material it started in. However that nucleus will not be able to decay in the same way again. (You can only smash a cup once, but you can go back over and stamp on all the bits again!) This means that the level of activity will gradually drop away over time as the nuclei all decay. For some elements, the activity will drop away to background very rapidly, for other elements activity drops more slowly. Half-life is a measure of how quickly the activity level drops.

See Activity.
Radiometric Dating
This is the name given to the general technique of using half-life to find out the age of samples of rock, fossil or material which is organic in nature. Carbon dating is used to date organic material up to about 30 000 years.
Radio Telescope
Astronomers use radio telescopes to help build up a picture of the Universe. Since radio waves can penetrate Earth's atmosphere, we can detect them by building large antennae or dishes. The radio part of the spectrum provides an alternative and complimentary view of the Universe - importantly radio waves convey information about much lower energy events than visible light does, much of what a radio telescope 'sees' will be not appear in visible light images.
Radio Transmitter
A source of radio waves. Radio transmitters are usually large vertical conducting masts which emit radio waves. Moving electrons in the metal of the mast radiate the radio waves.
This refers to the fact that the spectrum of distant galaxies is shifted towards the long wavelength 'red' end of the electromagnetic spectrum. (Redshift is caused by Doppler shift, beyond scope of standard grade physics). Bigger redshifts mean that the object is moving away from us faster. An astronomer by name of Edwin Hubble discovered that the more distant galaxies are moving away from us the most quickly (he measured the greatest redshifts for the farthest galaxies). This means that the Universe is expanding and leads directly to the currently popular Big-Bang theory of the origin of the Universe.
Reed Relay
A reed switch, controlled by an electromagnet. Relays can be used for remote control of electrical circuits in dangerous places. For example, a circuit within a nuclear reactor can be switched on or off with a reed relay. In general, relays can be used to switch high current circuits using low current circuits. In this way, control panels can be made safer for their operators because the more dangerous high current circuits can be separated from the control panel itself.
Reed Switch
A simple switch, controlled by a magnet. The reed switch can be designed either to make or break contact when a magnetic field is present. Reed switches can be used (with permanent magnets) as door switches, for example to switch on an oven light when the oven door is opened, or to switch off a microwave oven when the door is opened. In a reed relay, an electromagnet is used to control switch.
Reflection (Diagram)
All waves can be reflected from an obstacle - for example, water waves can reflect off a harbour wall and light waves can reflect off a mirror. The law of reflection is very simple:

The angle of incidence is equal to the angle of reflection.

Refraction (Diagram)
All waves can be refracted. Refraction means bending. Light rays refract when they pass through a medium of different density (e.g. when light travels from air into glass). When light passes into a region of increased density (e.g. air to glass) it bends towards the normal. When light passes into a region of reduced density, it bends away from the normal.
Renewable Energy Sources
Solar, wind, geothermal, tidal and wave power plants are examples of renewable energy resources. They will not run out, unlike fossil fuels. Currently political will exists to develop these sources and good progress has been made. Shetland already uses solar and wind plants to generate non-trivial power. There are plans to develop an off-shore wave power station too. For solar power, the following link is noteworthy, although a quick search on google will yield more. Solar power link.
The electrical resistance of a component (e.g. a lamp) is the component's opposition to the flow of current. The SI unit of resistance is the ohm (W). See also: Ohm's Law
A resistor is an electronic component which opposes the flow of current. See resistance and variable resistors.
A variable resistor, when used with just two of its terminals, it is called a rheostat. This arrangement is for controlling current. See also: potential divider.
Ring Circuit
This is a special type of parallel circuit, used in household wiring. The advantage of the ring circuit is that thinner, cheaper cable can be used. This is because there are two separate paths to any socket. Therefore, if each cable is capable of carrying up to 18 A, then the maximum current the ring can draw from the consumer unit is 36 A.
R.M.S. Voltage
This is a kind of average voltage of an a.c. source (averaged over a complete cycle). It is the effective voltage delivered by the source and can be used in power calculations (P=IV etc). Compare rms voltage with the peak voltage of an a.c. source.

In the UK, the rms voltage of a.c. mains is quoted at 230 V.

Rocket Engine
Rockets are propelled upwards by ejecting exhaust gases. Because of Newton's third law, as the rocket pushes the exhaust gases down, the exhaust exerts an equal and opposite upwards force on the rocket. It is much easier to understand rocket motion using the concept of momentum, which you will meet in Higher physics.
Rotor Coil
In an a.c. generator, the rotor coil is a rotating electromagnet, with a d.c. power supply. It replaces the permanent rotating magnets of a dynamo.


In astronomy and space science, a satellite is simply any object which is in orbit about a larger parent orbit. The force required to keep the satellite in orbit is the graviational attraction between the satellite and parent object. The moon and the ISS are satellites of Earth. Earth is a satellite of the Sun.
Planet Saturn has the sixth most distant orbit from our star the Sun. It is slightly smaller than Jupiter and is also a gas giant planet. Saturn's ring system is easily seen in a small telescope from Earth.
The second is the SI unit of time. It is defined as the time taken to produce 9 192 631 770 complete wavelengths of electromagnetic radiation from a particular event in cesium atoms.
Sievert (Sv)
The sievert is the SI unit of dose equivalent. Since 1 Sv is a massive dose of radiation, units of microsieverts are often used instead.
Scintillation Counter
These devices detect gamma rays and form part of the gamma cameras used in radioactive tracer examinations. Scintillations are flashes of light given out by fluorescent materials (e.g zinc sulphide. When radiation is absorbed by zinc sulphide, it causes the zinc sulphide to produce a small burst of light. These can be electronically counted.
Series Circuit
In a series circuit, each component is connected to the other to form a 'chain' between the terminals of the power supply. The rules for current, voltage and resistance in a series circuit are as follows:

Vsupply = V1 + V2 + V3 + ...

Isupply = I1 = I2 = I3 = ...

Rtotal = R1 + R2 + R3 + ...

The mnemonic VISA may help to remember the rules for voltage and current in series and parallel circuits:

VISA: Voltages In Series Add

Short Circuit
This is one of two types of circuit fault (the other being an open circuit). A short circuit fault is often caused by wires touching, allowing current to bypass a component in the circuit. Short circuit faults register on an ohmmeter with almost zero resistance.
This refers to a problem with the accommodation ability of an eye. Basically, the eye can focus on near-by objects with no problem, but can't properly focus on far-away objects. This is called myopia. This is because the retina is too far from the lens - so that the focal point falls short of the retina. Short-sightedness can be treated by wearing spectacles which are concave in shape and therefore help to focus the image properly by extending the overall focal length of the glasses and eye.
SI Units
SI stands for Systeme International. This is the standard system of units used worldwide and includes the metre, the second, the kilogram, the ampere, the kelvin, and the coulomb.

More information is available from the University of Alberta, CA or the University of Exeter, UK.

A solenoid is a coil of wire with an electrical current flowing through it. An iron core, inserted into the coil, has the effect of increasing the strength of the magnetic field produced. An increased current or a greater number of turns on the coil will also increase the strength of the magnetic field. See also: Electromagnet
Solar Cell
A solar cell is a photovoltaic semiconductor material which converts light energy into electrical energy. Solar cells are transducers.
A longitudinal wave which travels through a medium, usually air. Sound also travels through liquids (e.g. water) and solids (e.g. steel). The volume (loudness) of sound is measured using the decibel (dB) scale.
Specific Heat Capacity
The energy required to increase the temperature of 1kg of a material by 1 degree celsius. This is also the energy released during cooling. Water has a high specific heat capacity at 4200 Jkg-1 oC-1. Metals have much lower heat capacities. Physically, this is why metals heat up and cool down more rapidly than water - less energy is required to bring about a given change in temperature.
Specific Latent Heat of Fusion
This the latent heat per kilogram of material required to change its phase from solid to liquid (i.e. heat energy required to melt 1 kg of material). It is also the heat energy released (in joules) if 1 kg of material 'freezes' from liquid to solid.
Specific Latent Heat of Vapourisation
This the latent heat per kilogram of material required to change the material's phase from liquid to gas (i.e. heat energy required to boil or vapourise 1 kg of material). It is also the heat energy released (in joules) if 1 kg of material condenses from solid to liquid.
A device using the diffraction of light to split the visible spectrum up into the rainbow of colours. The spectroscope is used in astronomy to study the spectra of light from the stars.
(Plural: spectra) Usually refers to the 'rainbow' of colours seen by the human eye, Violet, Indigo, Blue, Green, Yellow, Orange, Red (in order of increasing wavelength). It is important to remember that this visible spectrum is only a tiny part of the full electromagnetic spectrum.
Speed is the rate at which distance is covered. The SI unit of speed is the metre per second. Other units include feet per second, kilometres per hour and miles per hour.

The simple formula for movement at constant speed is: distance = speed x time.

For waves, the formula v = f l can be used.

Speed Time Graph
This is a graph with time plotted along the horizontal axis, and with speed up the vertical axis. You need to be able to interpret these graphs for Standard grade. The area under the graph is the total distance travelled.
The electromagnetic spectrum includes all types of electromagnetic radiation(EMR). All types of EMR travel at the same speed: the speed of light, which is 3 x 108 ms-1. Starting with the lowest frequency and longest wavelength radiation, the spectrum consists of

The visible spectrum includes red, orange, yellow, green , blue, indigo, violet. Red light has a wavelength of approximately 700 nm and violet light's wavelength is roughly 400 nm. All other colours have a wavelength between these two limits.

A star is a hot ball of gas which shines by producing its own light. The closest star to Earth is called the Sun. The next closest is Proxima Centauri at around four light-years distant.Other types of stars include red giants, white dwarfs and neutron stars.
Stator Coil
In a dynamo, the stator coil is the the fixed coil which has an electric current (a.c.) induced in it by the motion of the nearby rotor (permanent magnet).
The stethoscope is a device which allows a doctor or nurse to listen to sounds coming from within a patient's body. The sounds can be used to diagnose medical conditions. The two bells of a stethoscope are connected to the earpieces via a flexible hollow tube. The bells can be selected by turning a valve which allows only sound from one of the bells to enter the tubing at a time. The open bell is used to listen to lower frequency (e.g. heart) sounds and the closed bell is used to listen to higher frequency (e.g. breathing sounds).
Streamlined objects are designed to reduce fluid drag forces. All rough or angular edges are removed, leaving only smooth curved surfaces. Fish, dolphins, falcons, air interceptors and racing cars are good examples of streamlined objects.
The Sun is our nearest star. It is a dim yellow dwarf star, not particularly big or bright by astronomical standards. It has a mass of 2x1030kg and diameter 110 times that of the Earth. As an example for comparison, Betelgeuse in Orion has around 20 times the mass of the Sun, and a diameter 290 times that of the Sun. Also in Orion, the bright star Rigel is intrinsically 60 000 times brighter than the Sun.


Communication (by any means) over a distance
A telescope is a device used in astronomy, including radio and optical telescopes. In a refracting telescope, an objective lens is used to collect light. The biggest optical telescopes are of the reflecting type, using a primary mirror to collect light. The aperture of the telescope determines how good it will be for observing faint objects. For use by a human, all optical telescopes also require an eyepiece lens.
Total Internal Reflection (TIR) (Diagram)
Total internal reflection occurs when light, instead of being refracted at a boundary between media of different densities, the light is totally reflected off the inside surface. The usual case is inside glass, where the light ray bounces off the inside of the glass instead of escaping out. This effect is the basis of fibre optics - used in communication and medicine.

Total internal reflection occurs only when the angle of incidence is above a certain value called the critical angle.

Thermistor (Diagram)
Thermistors are designed so that their resistance depends on temperature.

When subjected to low temperatures, the resistance of a thermistor is high.

When subjected to a high temperature, the resistance of a thermistor is low.

When two different metals are joined together, they will convert heat energy into electrical energy. This is the basis of the thermocouple. Thermomcouples are transducers.
Radioactive tracers are injected into a patient to examine the function of organs and blood supplies within the body. Technetium is often used as it emits gamma radiation which can be detected outside the patient's body. A gamma camera is used to detect the radiation as it travels around the parts of the body of interest.
Any device which converts one form of energy into another. For example, a microphone, a TV, a car or a solar cell.
A semiconductor device which can be used as an electronic switch (popular exam question). In standard grade physics you need to understand the operation of the NPN type transistor only.
A device which consists of two separate coils. Transformers only work with a.c. and can be used to step voltages up or down. A popular exam topic, transformers are well worth some study time.
Trough (Diagram)
This refers to waves. The trough of a wave is simply the part of the wave with the least (most negative) amplitude.
Truth Table
A table showing the output logic states of a logic gate or logic system for all given combinations of input logic states. Input states should be listed systematically to avoid confusion. As examples, have a look at the truth tables for the AND, OR and NOT gates. As a further example, here is the truth table for a 3 input XOR gate (exclusive OR), showing how to list the inputs in the correct order.
Tuner (Radio or TV)
The tuner in a radio or TV allows the user to select one channel from the many channels which arrive at the aerial. The output signal from the tuner section is then passed to the decoder.
Two-stage Rocket
Essentially, two rockets one on top of the other. The top rocket will not ignite until the first rocket is finished and jettisoned. The advantage over a big single stage rocket is that all the dead weight of the empty first stage can be cut away and left to fall back to Earth. This can greatly reduce the total mass of fuel required to reach orbit.


Ultrasound is sound which has a frequency (or pitch) greater than 20 kHz. This is above the maximum frequency which a human can hear. Many other animals however can hear sounds with a frequency above 20 kHz. Ultrasound can be so high pitched that no animal can hear it. Here are several application areas of ultrasound:
  1. Animal navigation
  2. Submarine navigation
  3. Fishing - detecting shoals
  4. Medical - looking inside the body without need of surgery. Often used during pregnancy.
All these applications rely on the fact that ultrasound is reflected when the density of the material through which they pass changes (in other words, they echo back when passing from skin to bone).
Ultraviolet (UV) radiation is invisible. It is a form of electromagnetic radiation, with a wavelength just shorter than that of visible violet light. UV radiation can be used to treat skin conditions like acne. Humans use UV light from the Sun to produce vitamin D3 (used in the kidneys and liver). UV light causes sun-tans, but too much can cause skin cancer.
The Universe in which we live consists of several hundred billion galaxies. These galaxies are grouped as superclusters and clusters. Astronomers currently favour the Big-Bang theory of the origin of the Universe, with recent results (of cosmic microwave background radiation) tending to support this view.
Planet Uranus has the seventh most distant orbit from our star the Sun. It is slightly smaller than Jupiter and Saturn and is also a gas giant planet. Uranus has very faint rings around it (not visible from Earth)


Variable Resistor
Variable resistors are resistors whose resistance can be altered, usually by means of a rotating or sliding contact. They can be used as potential dividers or rheostats.
The volt is the SI unit of voltage. In fact 1 V = 1 JC-1 (i.e. a charge of one coulomb will gain or lose one joule of potential energy when moved through a potential difference of one volt).
Planet Venus has the second most distant orbit from our star the Sun. It is slightly smaller than Earth. In many ways it is our sister planet - it receives similar heat energy from the sun (twice what Earth receives), with a similar internal structure to Earth. Its atmosphere is quite different, with 90 times the pressure of ours, and consisting of very corrosive gases. Its surface temperature can be well over 400 degrees celsius, due to a runaway greenhouse effect.
Voltage is an electrical concept. In circuits, voltage is a measure of the electrical energy available between two points. Voltage must be measured between two points in a circuit - e.g. "The voltage between point X and point Y is 3 volts". Voltages are measured using a device called a voltmeter. It is not necessary to break into a circuit in order to use a voltmeter.

It is wrong to say: "The voltage flows into the lamp". Voltage does not flow. It is current that flows. A difference in voltage between the terminals of a lamp causes current to flow through the lamp. (An analogy for voltage is the difference in heights between two points on a hillside. This difference in heights (voltage) causes water to flow downhill (electrical current). One would not speak of height flowing into a water-mill!).

Voltage, current and resistance are related through Ohm's Law

Voltage Divider
A simple circuit branch consisting of two resistors connected in series. The voltage accross the terminals of the branch is shared between the two resistors, with the bigger resistor getting the biggest share. Because the resistors are connected in series, the current through each is the same. The easiest way to calculate the voltage division for given resitances is to use division in ratios as follows:

Problem: Given resistors of 100R and 200R, connected in series, calculate the voltage division when connected to a 12V power supply.

Solution: 100:200 is equivalent to 1:2, so split the 12V supply voltage into 1+2=3 parts. Hence 1 part = 12/3 = 4 V and then 2 parts = 8 V. Do a quick check to see that 4+8=12V and then the answer can be given: the 100R resistor has a p.d. across it of 4V, the 200R p.d. is 8V.

Voltage Gain
Voltage gain has no units of its own. It expresses how many times greater the output signal voltage is (from some component) compared to the input voltage. A closely related concept is that of power gain. Voltage Gain = Output Voltage / Input Voltage.


The watt (W) is the SI Unit of power. It is equivalent to one joule per second (Js-1). Another common unit for power is the horsepower (hp). 1 hp = 746 W.
Wave (Diagram)
A travelling disturbance, carrying energy from place to place. Examples are water waves and sound waves where it is the movement of particles which constitutes the wave motion. In the example of light waves and other electromagnetic waves it is changing electric and magnetic fields which carry energy from place to place.
Wavelength, l (lambda) (Diagram)
This is the length of a wave between any two equivalent points. It is easiest to measure from crest to crest or from trough to trough. The SI unit of wavelength is the metre, although kilometres, millimetres, micrometres and nanometres are also used.

See also: cycle.

Weight is a force. The SI unit of force is the newton - not the kilogram!
Wind Turbine
When a force acts on a moving object, that force does mechanical work. The work done is defined by the equation:

Work Done = Force x Distance

The SI unit of work is the same as that of energy - the joule

The force only does work when it is at least partially aligned with the direction of movement. If the force acts at right angles to the direction of movement, then that force does no work. Work done can be used to calculate energy transferred. For example, if a box is pushed for 20 m with a force of 30 N, the pushing force does 600 J of work on the box (Work = 30 x 20 = 600 J). This work will either make the box accelerate or heat up due to friction (or both) - i.e. it will cause an increase in the kinetic energy of the box, or the heat energy within the box.

Note from the equation W = F d, the unit of one joule is equivalent to one newton.metre (1 J = 1 Nm)



X-rays were discovered accidentally by Wilhelm Röntgen in 1895. X-ray radiation is a part of the electromagnetic spectrum. With wavelengths ranging from about 0.001nm to about 100 nm, X-rays are much shorter than ultraviolet. They travel at the speed of light.

X-rays are produced in an X-ray tube (soft X-rays for diagnosis) or in a linear accelerator (hard X-rays for treatment).

X-rays can be detected using photographic film or scintillation counters.

In medicine, 'soft' X-rays can be used to take photographs of broken bones. Bone absorbs X-rays more than soft flesh and therefore shows up white on a photographic plate. A break in the bone will show up dark. (X-rays blacken the photographic chemicals on the film).

Alternatively, scintillation counters can be used to detect x-rays and display an image on a TV type screen. This method is used with barium meals for investigating the gut.

For a three dimensional view, computed tomography can be used where the X-ray source and detector rotate around the patient to build up a series of image slices.



This is a phrase often uttered by those people who finish reading the zenith entry below, having read the whole of this file.


The zenith is the point in the sky directly above an observer's head. This term is often used in astronomy.