Question

Which of the following is an example of blackbody radiation?
(A) Cooling of earth at night
(B) Solar radiations
(C) Heat currents from a black surface
(D) None of the above

Answer 1

Hint: It is necessary to have a clear idea about blackbody before solving the problem. So a black body radiation is in equilibrium with the agitation of molecules and electrons in the considered body. Or it can also be said as a pure thermal radiation.

Complete step by step answer: The most important thing to be understood before solving the problem is what is a black body and what do we mean by black body radiation.
So, we define a perfect blackbody as an object that absorbs and emits all the radiation of all wavelengths falling on its surface completely. And according to Stefan’s law we define black body radiation as the amount of radiant energy emitted per second per unit surface-area of a black body that is proportional to the fourth power of the absolute temperature of the body.
Now we will check each of the options whether they represent black body radiation or not.
Cooling of earth at night:- Earth surface warms up during the day from the radiations coming from the Sun. So, it absorbs the solar radiations during the daytime. Due to the terrestrial radiations as they are emitted from the surface. This behavior is similar to blackbody radiation and hence cooling of Earth at night can be called a blackbody radiation. Thus,

option A is a correct option.

Additional Information
Solar radiations:- Sun emits radiation of all wavelengths of the electromagnetic spectrum and hence behaves similar to the blackbody. Thus, solar radiations can be considered to be the black body radiation. Thus, option B is a correct option.
Heat currents from a black surface:- Black surface does not mean that it is a black body. Black surfaces do not emit radiations and hence heat currents from a black surface is not an example of blackbody radiation. Hence option C is not a correct answer.

Note: The absorbing power of a perfect black body is $1$ and reflecting power is $0$ . As the Sun sets in the absence of solar radiations, the Earth’s atmosphere starts cooling at night. This cooling takes place as the Earth’s surface emits the radiations to space. These radiations are longwave radiations and are known as terrestrial radiations