1905 Einstein demystified the Photoelectric Effect proving that light has a particle nature as well (and winning the nobel prize). The photoelectric effect occurs when light is shone on a piece of metal, and electrons are ejected from the surface of the metal.
Planck's quantum theory said that light was emitted in bundles of energy called photons. The energy of a photon is equal to Planck's constant (h=6.626x10-34 J/S) times the frequency of radiation (V) or E=hV.
According to Einstein photons that fall on a metal surface have their energy transferred to the electrons of the metal. If the photon has more energy than the electrons need to leave the surface (different metals have different requirements) the electrons are ejected immediately.
Einstein's photoelectric equation is: hv = W+1/2mv2
h is Planck's constant equal to 6.626x10-34 joules/second
v is the frequency of radiation in hertz. W is the Work function. It's the work necessary to remove electrons from the surface of the material. In the equation it is in joules. It may have to be converted from electron-volts (divide by 1.6x10-19).
1/2mv2 is the maximum kinetic energy of the ejected electrons (can also be expressed as Emax or maximum kinetic energy) in joules.
Applications of the photo electric effect include light sensing units. Garage door sensors work by sending an infrared light beam (from the sending unit) to a photo tube