# Why isn't the emissive power of a black body 1?

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

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

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., $$e=a=1$$