Question

The emissivity of a body varies from
A. zero to two
B. one to two
C. zero to one
D. always zero

Answer 1

Hint: Black body radiation emits the radiation with no matching ability with others. Emissivity is a measure of the efficiency in which a surface emits thermal energy. It is defined as the fraction of energy being emitted relative to that emitted by a thermally black surface (a black body). A black body is a material that is a perfect emitter of heat energy and has an emissivity value of 1.

Complete step by step answer:
Emissivity is defined as the ratio of the energy radiated from a materials surface to that radiated from a perfect emitter, known as black body, at the same temperature and wavelength and under same viewing conditions.
As we know that black body is the best emitter in the universe. Nothing matches the emissive power of a black body. Hence the ratio can never be 1.
Also the power is a scalar quantity and it doesn't have a negative sign.
Therefore from this we can say that the emissivity of a body varies from zero to one.

So, the correct answer is “Option C”.

Note: In this type of question we need to know about the emissive power of a radiating body and how they vary. To put it simply, a high emissivity object is better able to radiate energy than a low emissivity object. A blackbody would have an emissivity of 1, meaning that it would emit 100% efficiently. Lower emissivity objects would be less efficient emitters.