Question

The thermal radiation emitted by a body is proportional to Tn where T is its absolute temperature. The value of n is exactly 4 for
(a) a blackbody
(b) all bodies
(c) bodies painted black only
(d) polished bodies only

Answer 1

The thermal radiation emitted by a body is proportional to Tn where T is its absolute temperature. The value of n is exactly 4 for all bodies.

Explanation:

The Stefan-Boltzmann law defines that the total heat emitted by a surface is directly proportional to the fourth power of the temperature (absolute temperature). This law applies to all theoretical surfaces which absorb all radiation.

According to Stefan's Boltzmann law the energy of thermal radiation is given as

   $\mathrm{U}=\mathrm{e} \sigma \mathrm{AT}^{4}$

Where e = Emissivity

           A = Area of body

           T = Temperature of the body

The above law follows for all the bodies Hence the correct answer is Option (b)  all bodies

Answer 2

All objects with a temperature above absolute zero (0 K, -273.15 oC) emit energy in the form of electromagnetic radiation. A blackbody is a theoretical or model body which absorbs all radiation falling on it, reflecting or transmitting none.