Physics Definitions 2

The symbol for alpha radiation is α. Alpha radiation consists of slow moving helium nuclei. Alpha radiation will be stopped by a few centimetres of air, or a sheet of paper. It is a strongly ionising radiation. The helium nuclei are positively charged.

The symbol for beta radiation is β . Beta radiation consists of fast moving electrons. These electrons are produced as a result of transitions that take place in unstable nuclei. Beta radiation can be stopped by a few millimetres of aluminium. It is an ionising radiation, not as strongly ionising as alpha radiation

The symbol for gamma radiation is γ . 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 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 alpha radiation. It is released during the disintegration of an unstable nucleus.

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.

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 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.

This refers to a problem with the accommodation ability of an eye. The eye can focus on far away objects but can't properly focus on nearby objects. 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.

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.

• 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. The formula for this law is:
• F = ma

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.

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 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.

This applies to light rays passing through any system of mirrors or lenses. 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.

• 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.

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.

This refers to a problem with the accommodation ability of an eye. The eye can focus on nearby objects, but can't properly focus on far-away objects. 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.

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.