Key Terms

• Vector


  A quantity that has both a size and a direction
• absorption spectrum


  A spectrum of light (or electromagnetic radiation) that includes black lines. These are caused by some wavelengths being absorbed by materials that the light (or radiation) passes through
• acceleration


  A measure of how quickly the velocity of something is changing. It can be positive if the object is speeding up or negative if it is slowing down. It is a vector quanitity.
• Action - Reaction forces


  Pairs of forces on interacting objects. They are always the same size, in opposite directions, and on acting on different objects.
• activity


  The number of emissions of ionising radiation from a sample in a given time. This is usually given in becquerels (Bq)
• alpha particle


  A particle made up of two protons and two neutrons, emitted as ionising radiation from radioactive isotopes.
• amplitude


  The size of vibrations or the maximum distance a particle moves away from its resting position when a wave passes
• amplify


  To make bigger
• angle of incidence


  The angle between an incoming light ray and the normal
• angle of reflection


  The angle between the normal and a ray of light that has been reflected
• atom


  The smallest neutral part of an element that can take part in chemical reactions
• atomic number

  number of protons in the nucleus of an atom
• average speed


  the speed worked out from the total distance travelled divided by the total time taken for a journey
• background radiation

  Ionising radiation that is around us all the time from a number of sources. Some of it is naturally occurring, but some comes from human activities
• balanced forces


  When the forces in opposite directions are the same size so that there is a zero resultant force
• becquerel (Bq)


  The unit for the activity of a radioactive substance. One Bq = One Radioactive decay per second
• beta particle

  A particle of radiation emitted from the nucleus of a radioactive atom when it decays. It is a fast moving electron.
• braking distance


  The distance travelled by a vehicle while the brakes are applied to bring it to a halt
• centripetal force


  A force that causes objects to follow a circular path. The force acts towards the centre of the circle
• chain reaction


  The sequence of reactions produced when a nuclear fission reaction triggers one or more fissions
• cochlea


  The part of the ear that changes vibrations into electrical impulses
• elastic potential energy


  Stored or stretched energy that can allow an object to change back to its original shape
• electromagnetic waves


  a group of waves that travel at the same speed in a vacuum (speed of light) and are transverse
• electrostatic repulsion


  a force between two charges that have the same sign that pushes them apart
• emission spectrum


  A set of wavelengths of light or electromagnetic radiation showing which wavelengths have been given out by a substance
• external radiotherapy


  treatment of cancer by sending radiation into the body from the outside
• focal length


  the distance from a lens to the focal point
• focal point


  The point at which parallel light rays converge after passing through a converging lens (or spread out in a diverging lens)
• frequency


  the number of vibrations (or waves) per second measured in Hertz
• gamma ray


  high frequency, highly penetrating electromagnetic wave emitted from the nucleus of a radioactive atom
• Geiger-Muller (GM) Tube


  A device that can detect ionising radiation and is used to measure the activity of a radioactive source
• gravitational field strength


  A measure of how strong the force of gravity is somewhere. The units are newtons per kg (N/kg)
• half-life


  The average time taken for half of the radioactive nuclei in a sample of radioactive material to have decayed
• incident ray


  a ray of light going towards an interface or object
• inertial mass


  Ratio of force divided by acceleration.
• infrared radiation


  EM radiation that we can feel as heat
• infrasound


  Sound waves with a frequency below 20 Hz, which is too low for the human ear to detect
• instantaneous speed


  The speed at one particular moment in a journey
• interface


  The boundary between two mediums
• internal radiotherapy


  Treatment of cancer by putting a radioactive source inside the body