Question

A perfectly black body is one that ____________ .
A) Absorbs all the incident radiation
B) Reflects all the incident radiation
C) Transmits all the incident radiation
D) Reflects and transmits the incident radiation.

Answer 1

Hint: Every matter in this universe reflects some amount of radiation. Radiation is a form of energy. When radiation rays hit a surface some amount of energy is absorbed in the form of heat and the remaining gets reflected or refracted. Now it depends on the type of matter that how much it is absorbing and how much it is reflecting. A black body is a body that can absorb all the electromagnetic radiation without reflecting and transmitting the radiation.

Complete answer:
First let us understand what reflection, transmission, and absorption mean. Reflection refers to the nature of a surface of a matter which can change the direction of the incident ray of an electromagnetic ray with the same angle as the angle of the incident with the normal. Any surface reflects more radiation if it is smooth and flat. A rough surface reflects less amount of radiation and absorbs some amount of radiation in the form of heat.
Transmission is the nature of the body to let the radiation go through it. If a body can transmit all the radiation then it is a perfect transparent body that does not absorb the radiation in the form of energy
How does the absorption happen? Let us take an example of light which is electromagnetic radiation. Photon theory says that light is made of small packets called photons which possess the energy of light. When this ray of photons incident to the surface and if the surface is rough then all the photons do not get reflected. Some of them get absorbed by the body. That is why we feel a metal hot which is kept in sunlight.
If a body does not reflect or transmit any amount of radiation the body is called a black body or the ideal body. A black body absorbs all the incident radiation.

Hence the correct option is option A.

Note: In practical life there exist no such body that can absorb all the incident radiation but some surfaces are very close to the ideal body. Similarly, there does not exist a body that can reflect or transmit all the incident radiation. Every surface absorbs less or more radiation. It is obvious that if a black body absorbs all the incident radiation then it will not reflect or transmit any amount of radiation.