Photoelectric effect

The photoelectric effect is a phenomenon in which electrons are emitted from matter (metals and non-metallic solids, liquids or gases) as a consequence of their absorption of energy from electromagnetic radiation of very short wavelength, such as visible or ultraviolet light.

In 1902, Philipp Lenard observed that the energy of individual emitted electrons increased with the frequency of the light. This appeared to be at odds with Maxwell's wave theory of light, which was thought to predict that the electron energy would be proportional to the intensity of the radiation.

When Lenard applied a retarding potential to the electrons he found that the 'stopping voltage' depended both on the frequency of the light and the material of the electrode, but did not depend on the intensity of the light. Brighter light did not increase the kinetic energy of the electrons.



In 1905, Einstein solved this apparent paradox by describing light as composed of discrete quanta, now called photons, rather than continuous waves. Einstein theorized that the energy in each quantum of light was equal to the frequency multiplied by a constant, later called Planck's constant.

E = hf - W
where E = electron kinetic energy
f = light frequency
and W = work function
The Work Function is the energy required to eject the electron.

W = hf0   where f0  is the threshold frequency.