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 slowingdown. It is a vectorquanitity.
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 nuclearfission reaction triggers one or more fissions
cochlea
The part of the ear that changes vibrations into electricalimpulses
elastic potential energy
Stored or stretchedenergy 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 twocharges that have the same sign that pushes them apart
emission spectrum
A set of wavelengths of light or electromagneticradiation 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 focalpoint
focalpoint
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) persecond measured in Hertz
gammaray
high frequency, highly penetrating electromagnetic wave emitted from the nucleus of a radioactiveatom
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 radioactivenuclei 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