Question

Good absorbers of heat are
(A). Poor emitters of heat
(B). Good emitters of heat
(C). Only black bodies
(D). None of these

Answer 1

- Hint: According to Kirchhoff’s Law of Thermal Radiation: the coefficient of emission of a body is equal to its coefficient of absorption. This implies that the amount of heat absorbed, and heat radiated by a body are always equal.

Complete step-by-step solution -

Transfer of heat from a source to an object without any actual movement of the source or the object takes place in the form of radiation. Heat transferred by radiation does not require any material medium. For example, the Sun’s heat is received by us through outer space in the form of radiation. Whenever a body is heated it emits radiations of different frequencies in all directions.
 Kirchhoff's Law states that the power radiated by an object must be equal to the power absorbed. In other words, we can say that, for an arbitrary body at thermal equilibrium, emissivity is equal to its absorptivity. Which means that the good absorbers of heat are also good emitters of heat.
Hence, the correct answer is B. Good emitters of heat.

Additional Information:
A blackbody is a well-known example of a perfect absorber and a perfect emitter. The radiation emitted by a black body is called black body radiation. The discovery of the blackbody was the first evidence of the dual nature of electromagnetic radiation(light).

Note: Students must know that the most realistic version of a blackbody is a hole in a black insulated enclosure or a cavity. A beam of light entering this enclosure would undergo multiple reflections on the inner walls and would be completely absorbed and never get reflected back outside.