The emission spectrum shows the colours created when light produced by hot substances is passed through a prism or through a diffraction grating.

Hook's Law states that the extension of a spring is directly proportional to the force extending it.

A photoelectric cell is apparatus that we use to demonstrate that light can liberate electrons from a metal surface.

Plastic deformation occurs when a material is stretched beyond its elastic limit and will not return to its original shape after the deforming force is removed.

Population inversion occurs when the number of metastable neon atoms outnumbers neon atoms in the ground state.

Quenching is the process whereby steel is heated until it is red hot and then plunged into cold water. This process makes the steel brittle.

Spontaneous emission occurs when excited atoms lose their energy through random de-excitation and then give off photons of light.

Tempering is a process whereby annealed steel is reheated and then quenched in cold water. This makes the steel springy.

Threshold frequency is the frequency of the incident light during the photoelectric effect that is sufficient to emit electrons from the photoelectric cell.

Work function refers to the forces of attraction in a metal that determine the energy needed for the electrons to be ejected during the photoelectric effect.