Question

Which of the following law states that “good absorbers of heat are good emitters”?
(A) Stefan’s law
(B) Kirchhoff’s law
(C ) Planck’s law
(D) Wein’s law

Answer 1

Hint: For a given temperature the absorption coefficient of the body is equal to the emission coefficient of the body according to Kirchhoff’s law of radiation. According to Stefan’s law for a blackbody the fourth power of absolute temperature is proportional to the radiated power density. Where the constant of proportionality is called Stefan’s constant. In Planck’s law it is the mathematical relation between the radiation emitted by a blackbody at equilibrium temperature. Wein’s displacement law is a law connecting the relationship between wavelengths of emitted radiation and temperature of a blackbody.

Complete step by step answer:
According to Kirchhoff’s law of radiation the good heat absorbers are also good heat emitters. Thus for a given temperature the absorption coefficient of the body is equal to the emission coefficient of body.
According to Stefan’s law for a blackbody the fourth power of absolute temperature is proportional to the radiated power density. Where the constant of proportionality is called Stefan’s constant. This law is only applicable to blackbodies. To calculate the surface temperature of stars this law is used. In case of a blackbody it can be described as a perfect absorber, but not a perfect emitter. That is, when a radiation is incident on a blackbody it absorbs internally. Thus a blackbody can absorb wavelengths of all radiation and any angle of incidence. Thus the concept of blackbody is a useful application in the study of radiation.
According to Planck’s law when there is no flow of matter or energy between the system and its surrounding , the spectral density of radiation emitted by a blackbody in thermal equilibrium at a given temperature. Hence this law gives a mathematical relation between the radiation emitted by a blackbody at equilibrium temperature.
Wein’s law otherwise called Wein’s displacement law is a law connecting the relationship between wavelengths of emitted radiation and temperature of a blackbody.

So, the correct answer is “Option B”.

Note: According to Kirchhoff’s law of radiation the good heat absorbers are also good heat emitters. Stefan’s law is only applicable to blackbodies. To calculate the surface temperature of stars this law is used. In case of a blackbody it can be described as a perfect absorber, but not a perfect emitter. The concept of blackbody is a useful application in the study of radiation.