When a black body is heated, it emits thermal radiations of different wavelengths or frequency. To explain these radiations, Max Planck put forward a theory known as Planck's quantum theory. The main points of quantum theory are

i) Substances radiate or absorb energy discontinuously in the form of small packets or bundles of energy.

ii) The smallest packet of energy is called quantum. In case of light the quantum is known as photon.

iii) The energy of a quantum is directly proportional to the frequency of the radiation. `E prop nu` (or) `E = h nu` were `nu` is the frequency of radiation and `h` is Planck's constant having the value `6.626 xx 10^(-27)` erg-sec or `6.626 xx 10^(-34)` J-sec.

iv) A body can radiate or absorb energy in whole number multiples of a quantum. Hence it can be `h nu`, `2h nu`, `3h nu` ........... `nh nu`. where `n` is a positive integer.

v) Energy of photons can also be represented by `((nhc)/lambda) ` Joules, Where,

`n` = number of photon

`h = 6.0626 xx 10^(-34)` J-s

`c = 3 xx 10^8` m/s

`lambda` : wavelength in 'm'

In eV units

`E _(text(photon)) = (12400)/(lambda(text(in) overseto(A))) eV`

`1 eV = 1.6 xx 10^(-19)` Joules