Photoemission

Energy of photon = Energy needed to remove and electron + Kinetic energy of emitted electron

Or

hf = hf0 + ½ mv_max²

Or

hf = ø + E_k

h - Planck's constant
f0 - threshold frequency for the photoelectric effect to occur
ø - work function
E_k - maximum kinetic energy observed

When this is not true is may be because an excess of energy means some energy is absorbed as heat or emitted as radiation.