Photoelectric Effect:

In 1887, Hertz observed that when a light of appropriate wavelength falls on a clean surface of metal plate placed in an evacuated vessel a stream of electrons is emitted. These electrons are called as photoelectrons because these are emitted due to light. As moving electrons constitute electricity, the phenomenon of emission of photoelectrons is referred to as the photoelectric effect.  Alkali metals with loosely held electrons are better photoelectric metals. Like blackbody radiation, the existence of the photoelectric effect and the related experimental observations could also not be explained by classical physics. These and some other experimental phenomenon could be explained in terms of the quantum mechanical model. Let us know about it.

In the quantum or particle model, a radiant energy is considered as stream of discrete particles or packets of energy called as quanta (singular, quantum) whose energy is proportional to its frequency. The two are related as,

The quantum of light was later called a photon.

E = hν

Alternatively, the energy of the quantum can be related to the wavelength or wave number as follows

E = h c/λ  

or E = hc

 It is important to remember that energy of the photon is not same as the radiant power. The radiant power includes the energy of the photon as well as the number of the photons in the radiation.

Let us take an instance to understand the application of the relationship among the energy and other characteristics of the radiation.