The Photoelectric Effect
Einsten was awarded the Nobel Prize in Physics especially for the discovery of law of Photoelectric Effect. It established the wave-particle duality of light.
When light with sufficient energy hits a metal surface, an electron that was previously bound to the metal is ejected. The energy from the light photons is transferred to the electron. If the energy is enough to make the electron jump out of the atom, it is ejected and is referred to as a photoelectron. The minimum amount of frequency at which the required energy is achieved is termed as threshold frequency(µ0).
At a frequency µ>µ0, the electron is ejected out with a certain kinetic energy which increases with increase in frequency of the exposed light.
A notable observation was that even though the number of electrons ejected depends on the brightness of light, their kinetic energy does not. For example, red light [µ = (4.3 to 4.6) × 1014 Hz] of any brightness (intensity) may shine on a piece of potassium metal for hours but no photoelectrons are ejected. But, as soon as even a very weak yellow light (µ = 5.1–5.2 × 1014 Hz) shines on the potassium metal, the photoelectric effect is observed. The threshold frequency (µ0) for potassium metal is 5.0×1014 Hz.