Radiation, as we know, is the method of heat transfer when the transmitting medium is air or Vacuum in other words the material medium is not needed in case of radiative heat transfer.

Molecules of the material medium can absorb, reflect and transmit. If the material medium is a vacuum then there will no absorption in the molecules thus total reflectivity results in Vacuum.

Therefore the radiation heat transfer is more effective as compared to convective and conduction transfer.

The surface that absorbs the incident radiation and reflects none is called a black body surface.

The Stefan –Boltzmann law for Black body surface states that the rate of radiation energy from the surface per unit area is proportional to the fourth power of the temperature of the body.
Q=σ AT4
Where q is the rate of heat transfer
σ= Stefan’s constant
T= temperature of the surface

Now if we consider that the surface of one black body is with temperature T1 and the other black body is at temperature T2 then the net heat radiated transfer is given by
Q=σ(T14 -T24)A

Black body is a perfect radiator. Real bodies, however, do not act as a perfect radiator and emit at a lower rate.

To take into account the real nature of the radiant bodies, a factor, called emissivity, is introduced. Emissivity is defined as the ratio of the emission from a real “grey” surface to the emission from a perfect “black” surface. It is denoted by a Greek symbol epsilon ε.
Q=ε σ (T14 -T24)A

Please comment below for any question!