Every object in the universe radiates heat energy.

The sun is the biggest source of heat radiation from which the heat energy reaches to the earth.

The electric bulb and tubelight also radiate heat energy.

When we stand beside a fire, we feel hot because of heat radiated by the fire.

And as every object radiates heat energy, but their rate of heat transfer is different.

There is a law known as Stefan Boltzmann's law which tells about the rate of heat transfer in radiation.

Let's study Stefan Boltzmann law.

It states that " The rate of heat transfer by radiation is directly proportional to the area of cross-section and the fourth power of the absolute temperature" of the body.

And, to remove the proportionality sign we have to use a constant ($σ$), which is known as the Stefan-Boltzmann constant.

The value of this constant is $5.67×10_{−8}Wm_{−2}K_{−4}$.

This expression is only applicable to black bodies or say for a perfect radiator.

The heat transfer rate for the normal bodies are only a fraction of the value obtained by this expression.

For a non-black body, the expression is modified and $ϵ$, the emissivity of the body is introduced.

Emissivity is defined as the ratio of emissive power of a body to the emissive power of the black body.

The emissive power of the black body is one and for another body, it will be less than one.

Hence, for a perfectly black body, we have found out the heat transfer rate is maximum.

Revision

This is the expression to find the heat transfer rate by radiation of the blackbody.

But the heat transfer rate for a non black body is given as,