Question

An ideal black body at room temperature is thrown into a furnace. It is observed that
A. initially it is the darkest body and at later times the brightest.
B. It is the darkest body at all times.
C. it cannot be distinguished at all times.
D. Initially it is darkest and at later times it cannot be distinguished.

Answer 1

Hint: An ideal black body is that body that absorbs the radiations of all the possible wavelengths that are incident on it. When the temperature of the black body is equal to the surrounding temperature, it emits all the absorbed radiation. Hence, its intensity is maximum.

Complete step-by-step solution:
Let us first understand what is meant by a black body. Then we will be able to answer what will happen to the ideal black body when it is thrown into a furnace.
Every object in the universe absorbs, reflects, and transmits when heat radiations (also known as thermal radiation) are incident on them. Due to this the temperature the object rises.
A black body is that body that absorbs all the thermal radiation that falls on it and does not reflect any of the radiations. Thermal radiations are electromagnetic waves. Therefore, a black body absorbs the thermal radiations of all the possible wavelengths.
Since a black body does not reflect radiations, it appears black in color.
Once the temperature of the black body becomes equal to the surrounding temperature, the black begins to emit radiations of all the possible wavelengths. This means that a black body emits maximum radiations. Hence, the intensity of emission is maximum for a black body. Thus, it will appear as the brightest object.
Therefore, when an ideal black body is thrown into a furnace, initially it is the darkest object since it absorbs all the surrounding radiations. Later, when the temperature of the black body becomes equal to the temperature of the furnace, the black body will emit radiations of all the possible wavelengths. Thus making it the brightest object.
Hence, the correct option is A.

Note: Let us stress on the term used for an ideal black body. We discussed that a black body is a body that absorbs the radiations of all the possible wavelengths and appears the darkest (black). However, a perfect black body does not exist. This means that there is no such body that absorbs and emits all the radiations that fall on it.
Hence, when we consider a perfectly black body, we say that this body is an ideal body.