My text book has a question which says that the emissive power of a black body isn't one but the answer states that the absorptive power is 1, considering that $$e=a \tag{Kirchoff's law}$$

and a black body is defined as an object which has $$e=1$$ Then why isn't $a=1$

  1. Choose correct options

(a) Good absorbers of a particular wavelength are good emitters of same wavelength.This statement was given by Kirchoff.

(b) At low temperature of a body the rate of cooling is directly proportional to temperature of the body.This statement was given by Newton.

(c) Emissive power of a perfectly black body is 1

(d) Absorptive power of a perfectly black body is 1

The answer is given as (a,d)

Waves and Thermodyanamics by DC Pandey 15th edition

  • $\begingroup$ Can you quote the textbook question for context and accuracy? $\endgroup$
    – J. Murray
    Nov 11, 2018 at 15:18
  • $\begingroup$ @J.Murray Done.. $\endgroup$
    – hjhjhj
    Nov 11, 2018 at 15:29

2 Answers 2


For blackbody $a=e=1. a=1$ implies that the blackbody absorbs all radiation falling on it. For anything that is not a blackbody, $0≤e=a<1$


The emissivity ($e$) of a perfectly black body is 1. Emissivity is the ratio of energy emitted by a body to the energy emitted by a black body. The emissive power is the energy emitted per unit area per unit time. The emissive power of a black body is not 1, and it varies with temperature.

It is true that emissivity = absorptive power = 1 for a black body, i.e., $e=a=1$


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