PH2 definitions - photons, lasers etc.

The emission of a photon from an excited atom.

Triggered by an incident photon of energy equal to the energy gap between the excited state and the lower energy state of the atom.

Emitted photon has same frequency, phase, direction of travel and polarisation as Incdent photon.

When light approaches the boundary between two media from the 'slower' medium, the critical angle is the largest angle of incidence for which refraction can occur. The refracted wave is then travelling at 90° to the normal.

A situation where a higher energy state in an atomic system is more heavily populated than a lower energy state (i.e. a less excited state or the ground state) of the same system.

Feeding energy into the amplifying medium of a laser to produce a population inversion.

An electron or electron neutrino.

Particles consisting only of quarks and antiquarks, (only quarks and antiquarks feel the strong force