Question

A perfect black body is a
(This question has multiple correct options)
(A) Perfect absorber of radiations
(B) Perfect emitter of radiations
(C) Perfect reflector of radiations

Answer 1

Hint Absorptivity is the fraction of radiation absorbed by a body. Reflectivity is the fraction of radiation reflected by a body. Emissivity is a dimensionless quantity that helps in comparing the radiation-emitting capability of a body; its value ranges between zero to one for the black body; its value is one.

Complete Step-by-step answer
A body that can absorb the radiation of all frequency incidents on it irrespective of the angle of incidence is known as a black body.
A good absorber is also a good emitter. Therefore black bodies are a good emitter and also a good absorber of radiation.

Hence the correct answer is (A) Perfect absorber of radiations and (B) Perfect emitter of radiations.

Additional information
The concept of a perfectly black body was firstly introduced by Kirchhoff. A black body is an ideal emitter which means that they emit radiation with an energy that is greater than the radiation produced by any other body at the same temperature. They emit energy isotropically i.e. independent of direction hence energy released in every direction is the same. Examples of a black body are a cavity with a hole, near black materials, stars, and planets, black holes, etc.
One must remember that a perfectly black body is currently a hypothetical phenomenon the above examples show that these bodies are considered as bodies with similar properties to an ideal black body.

Note
Black body good absorbers this means that they absorb all the radiation falling on them so they are poor reflectors and transmitters of radiation, therefore, option (C) Perfect reflector of radiations is an incorrect option.