Question

A hot body will emit radiations more rapidly if its surface is
a) black and polished
b) white and polished
c) black and rough
d) white and rough

Answer 1

Hint: A rough surface is not that smooth and hence does not absorb all the much of the heat incident on it whereas a smooth polished surface does. Similarly, a black colored body absorbs more of the incident radiations on it compared to any color. Hence further to determine which body will emit radiation more rapidly will use Kirchhoff law of heat radiation

Complete answer:
To begin with let us first define the Kirchhoff’s law of heat radiation.

It states that at any given temperature, the ratio of the emissive power to the absorptive power corresponding to the certain wavelength is constant for all bodies. What we can conclude from this is a good absorber of heat is a good emitter of heat radiation as well.

A black polished surface absorbs maximum of the radiation incident on it. Hence By Kirchhoff’s law of heat radiation we can conclude that a perfect black body with the polished surface will emit radiation more rapidly than any other body.

So, the correct answer is “Option A”.

Note:
It is to be noted a good absorber of heat is a poor reflector of heat. Hence white surface is a good reflector of heat and that’s the reason why people prefer white colored clothes in the summer season. Black color surfaces being a good absorber of heat, the walls of the solar cooker are painted black from inside, but black is also a good emitter hence to trap the heat radiations inside they are fitted with glass.