Question

An ideal black body at room temperature is thrown in 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 the darkest body and at later times it cannot be distinguished.

Answer 1

An ideal black body is such that it absorbs radiations of all possible wavelengths that fall on it. When the black body attains a temperature that is equal to the surrounding temperature, it emits all absorbed radiation. Hence, the intensity of radiation is maximum.

Complete answer:
When the black body is thrown into the furnace, it absorbs the thermal radiation (heat) from the furnace. Initially, the black body does not reflect any of the radiation that it absorbs. The temperature of the black body keeps on rising until it becomes equal to that of the furnace. When the temperature of the black body becomes equal to the furnace, it emits radiations of all possible wavelengths. This makes it the brightest object.

The answer is option A.

Note:
B. It is the darkest body at all the times – The black body emits radiation and appears bright when it attains a temperature equal to that of the furnace. Hence, it is not the darkest body at all times.
C. It cannot be distinguished at all the times – The black body will appear black until it attains temperature equal to that of the furnace and then appear brighter than the furnace as it emits maximum radiation. Hence, it is distinguishable at all times.
D. Initially it is the darkest body and at later times it cannot be distinguished – The black body, when it attains temperature equal to that of the furnace, emits radiation of all possible wavelengths. Hence, the black body emits radiations of maximum intensity, making it the brightest body. Therefore, it can easily be distinguished later.