Some phenomena like Photoelectric effect, Compton effect, Raman effect could not be explained by wave theory of light. Therefore, quantum theory of light was proposed by Einstein. According to quantum theory of light “light is propagated in bundles of small energy, each bundle being called a photon and possessing energy.”
Momentum of photon,
Rest mass of photon = 0
Dynamic or kinetic mass of photon,
The number of photons in a source of monochromatic radiation of wavelength λ and energy W or power P.
The phenomenon of emission of electrons from a metallic surface by the use of light (or radiant) energy is called photoelectric effect. The phenomenon was discovered by Lenard. For photoelectric emission, the metal used must have low work function, e.g., alkali metals. Caesium is the best metal for photoelectric effect.