A body has a black spot on it. When the body | vedclass

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(C) According to Kirchhoff's law of radiation,the ratio of emissive power $(e)$ to the absorptive power $(a)$ for a body at a given temperature and wa

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Kirchhoff's law

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(C) According to Kirchhoff's law of radiation,the ratio of emissive power $(e)$ to the absorptive power $(a)$ for a body at a given temperature and wavelength is constant and equal to the emissive power of a perfectly black body at that temperature. Mathematically,$\frac{e}{a} = E_{\lambda}$,where $E_{\lambda}$ is the emissive power of a black body. This implies that $e \propto a$. $A$ black spot on a body has a higher absorptive power $(a \approx 1)$ compared to the rest of the surface. Since it is a better absorber,it is also a better emitter of radiation at high temperatures. Therefore,when the body is heated and placed in a dark room,the black spot emits more radiation and appears to glow more brightly. Thus,the correct option is $C$.

$A$ body has a black spot on it. When the body is heated and taken into a dark room,the black spot glows more brightly. This can be explained by which of the following?

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If $E$ is the total energy emitted by a body at temperature $T \ K$ and $E_{max}$ is the maximum energy emitted by it at the same temperature,then:

$Assertion :$ $A$ body that is a good radiator is also a good absorber of radiation at a given wavelength.
$Reason :$ According to Kirchhoff's law,the absorptivity of a body is equal to its emissivity at a given wavelength.

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