Consider an electromagnetic radiator enclosed in a fixed volume 'V' at a fixed temperature T. One can obtain such a system by making a cavity in any material and then heating the material to temperature 'T'. The atoms of the walls of the cavity constantly emit and absorb radiation. At an equilibrium we will have a cavity filled with electromagnetic radiation. Such cavity is called black body cavity and the radiation within the cavity is called black body radiations.

According to quantum mechanical theory an electromagnetic wave may be regarded as massless particle of spin angular momentum ℏ and with energy and momentum are function of wavelength.

These massless particles are called Photons.

The total energy of the state of electromagnetic field. Photons with momentum ℏk and polarization vector k is given by

Equation vi is known as energy density due to photons of frequency w and which is known as Plank's radiation form.

Pressure exerted by Photon gas

This note is taken from Statistical Mechanics, MSC physics, Nepal.

This note is a part of the Physics Repository.

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