Question

Absorptive power of a body is defined as the ratio of
A. The amount of heat energy incident on it to the amount of heat energy absorbed in a wavelength range.
B. The amount of heat energy absorbed to the amount of heat energy incident on it in a wavelength range Tension and surface tension.
C. The amount of heat energy absorbed to the amount of heat energy radiated by it in a wavelength range Tension and surface tension.
D. None of the above.

Answer 1

Hint: A body when subjected to radiations, absorbs it, reflects it and radiates some of it. Try to understand the words ‘absorptive power’ and draw a relation if the fact is not memorized.

Complete step by step answer:
Electromagnetic (EM) radiations or EM waves are transverse in nature, that is they do not require a medium to travel. As the direction of propagation of the motion is perpendicular to the plane in which the components oscillate. Therefore, EM radiation like light travels in empty space and vacuum to reach Earth.
Although when subjected to an object to a radiation, they behave a little differently. The speed of the radiation in a medium, although constant, in a material or a body depends on a number of factors such as absorptive power, emissive power etc.
If a body is subjected to a certain amount of radiations. The body absorbs some part of it, reflects some part of it and transmits the rest. Refer to the figure. A perfect black body is defined as one which absorbs all the radiations indecent on it.

The absorptive power of a body is defined as the ratio of the energy absorbed for a given time to the energy incident on it for the same amount of time. It measures the amount of heat absorbed by an object. Since, a black body absorbs all the incident energy, its absorptive power is 1.

\[\text{absorptive power(}a)=\dfrac{\text{ Amount of energy absorbed }}{\text{ Amount of energy incident}}\]
Whereas, emissivity is defined as the ratio of the energy emitted by the body to the energy emitted by a black body.
Therefore, the answer to this question is option B.

Note: Option A. is incorrect as it defines the quantity that is the reciprocal of absorptive power. The energy that a body emits or radiates is equal to the absorbed energy when the body is in thermal equilibrium. Option C. defines the ratio of the amount of absorption to the amount of radiation which is not absorptive power.