# Nuclear Definitions:
- _Isotope_ - atoms of the same element with the same number of protons but a differing number of neutrons
- _Decay constant_ - The probability of a nucleus day per unit time
- _Activity_ - The number of nuclei that decay per unit time
- _Half life_ - The time taken for the activity of a sample to half
- _Atomic mass unit_ - 1/12 mass of C-12 atom
- _Mass difference_ - The difference in mass between the nucleus and the sum of it's constituent nucleons
- _Binding energy_ - The energy required to separate the nucleus into it's constituent nucleons
- _Binding energy per nucleon_ - The average energy per nucleon to remove all the nucleons from a nucleus
- _Fission_ - A large or heavy nucleus is bombarded with neutrons causing it to break up into two smaller nuclei and release two-three neutrons
- _Fusion_ - Two small or light nuclei overcome the electrostatic repulsion between them (after being given sufficient kinetic energy) and combine to form a larger nucleus
- _Thermal neutrons_ - Neutrons that have low energies or speeds (e.g. 0.03eV)
- _Chain reaction_ - Fission reaction is induced by neutron bombardment, which then releases neutrons which then go on to cause further fissions
- _Critical mass_ - The minimum mass of fissile material for a self-sustaining reaction to be maintained

# Thermal Definitions:
- _Specific latent heat of vaporisation_ - The energy required to change 1kg of liquid into 1kg of gas
- _Specific latent heat of fusion_ - The energy required to change 1kg of liquid into 1kg of solid
- _Specific heat capacity_ - The energy required to raise the temperature of 1kg of a substance by 1K
- _Random motion (wrt particles)_ - particles have a range of speeds with no preferred direction
- _Boyle's Law_ - Pressure is inversely proportional to volume at a constant temperature
- _Charle's Law_ - Volume is proportional to temperature at a constant pressure
- _Pressure-temperature Law_ - Pressure is proportional to temperature at a constant volume

# Astrophysics Definitions:
- _Principal focus_ - The point through which all light parallel to the axis of the lens passes through
- _Focal length_ - The distance between the centre of the lens and the principal focus (focal point)
- _Spherical aberration (lens)_ - Where parallel rays furthest from the principal axis are brought to focus at different points
- _Spherical aberration (mirror)_ - Where parallel rays furthest from the principal axis are brought to focus closer to the reflector than the rays closest to the principal axis
- _Chromatic aberration_ - When different wavelengths are refracted different amounts resulting in different focal lengths for different wavelengths
- _Rayleigh criteria_ - two point objects can be just resolved when the central maximum of the diffraction pattern of one object lies over the first minimum of the diffraction patter of the other.
- _Airy disk_ - The bright central maximum of the diffraction pattern when a light from a point source is shone through a circular aperture
- _Quantum efficiency_ - The proportion of incident photons on the CCD that release electrons
- _Parsec_ - The distance to an object subtending 1 arcsecond to the radius of the Earth's orbit
- _Light year_ - the distance light travels in a vacuum in 1 year
- _Astronomical unit_ - The mean distance between the Earth and the Sun
- _Absolute magnitude_ - The brightness of object would be if it was 10Pc from Earth
- _Apparent magnitude_ - The brightness an object as seen from Earth
- _Lambda max_ - The wavelength at which maximum emission occurs
- _Intensity_ - The energy arriving every second on a 1m^2 surface, if the radiation was arriving incident on the surface.
- _Supernova_ - Absolute magnitude of a star rises rapidly due to it exploding
- _Event horizon_ - The boundary where the escape velocity = c.
- _Doppler effect_ -  Change in wavelength due to relative velocity between observer and source.
- _Red shift_ - Increase in wavelength due to relative recessive velocity between observer and source