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`        justify the statement-"A good absorber is a good emitter".`
7 years ago

Askiitians Expert Himanshu Geed - IIT Delhi
31 Points
```										Dear Pramod,Perhaps the most fundamental conceptual way to approach this question is  to observe that a hot object placed in a room must ultimately come to  thermal equilibrium with the room. The hot object will initially emit  more energy into the room than it absorbs from the room, but that will  cause the temperature of the room to rise and the temperature of the  object to drop. But when they reach the same temperature, we can  conclude that the amount of energy absorbed on average is exactly the  same as the energy emitted.
Consider a hollow enclosure with its walls maintained at temperature T.  The enclosure is filled with thermal radiation. Let I be the energy per  unit time falling per unit area of any (imaginary) surface in the  enclosure. I is called irradiance in the enclosure. Imagine now that a  black body at the same temperature T is introduced in the enclosure. The  body will be in thermal equilibrium, i.e., the radiant energy per unit  time per unit area emitted by the black body (EB) equals the  radiant energy per unit time per unit area absorbed by it. That is,

since aB = 1 for the black body.
Next, introduce a non-black body (a < 1) at the temperature T  inside the enclosure. Thermal equilibrium now demands that
E = a I
Where E is the radiant energy per unit time per unit area emitted by  the (non-black) body. It follows from Eq.(1) that

But in terms of emissivity e already defined

which shows
a = e
That is, the absorptivity of a body equals its emissivity. This is  known as Kirchhoffs Law. A good absorber is thus a good emitter.
Please feel free to post as many doubts on our discussion forum as you can. If you find any question Difficult to understand - post it here and we will get you the answer and detailed solution very quickly. We are all IITians and here to help you in your IIT JEE preparation. All the best  !!!Regards,Askiitians ExpertsHimanshu Geed
```
7 years ago
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