Now,

(5.207)

or

(5.208)

Also

(5.209)

or

(5.210)     (5.211)

Equation (5.211) are known as Plank’s law of thermal radiation in terms of frequency and wave-length respectively.

When ,Eq. (5.211) reduces to:

(5.212)

This is known as Rayleigh-Jeans formula.

When , Eq. (5.211) becomes

(5.213)

This is known as Wien’s law.

Again

(5.214)

where

(5.215)

The Eq. (5.214) is known as the Stefan-Boltzman law and σ is called the Stefan-Boltzman constant.