Photons
particles have no
rest mass - composed of pure electromagnetic energy - the absorption and emission of photons by atoms and
molecules is the only
mechanism for atoms and molecules can gain or lose energy
Quantum mechanics
absorption and emission are quantized - i.e. discrete
process of gaining or losing
energy in strict units of energy - i.e. photons of the same energy (multiple units are referred to as
electromagnetic radiation)
Energy
of a photon
can be computed from its frequency (n)
in hertz (Hz) or its
wavelength (l) in meters from
»E=hn and E=hc/l
