Question

Emissivity can be defined as the ratio of
A. emissive power of real body to the emissive power of black body
B. emissive power of black body to the emissive power of real body
C. reflectivity of real body to emissive power of black body
D. reflectivity of black body to emissive power of real body

Answer 1

A body can be termed as a black body when it absorbs all the rays of the incident radiant energy falling on its surface. The emissive power of a body is the quantity of radiant energy emitted from the surface of that body in a unit time and in unit area in all the possible directions.

Complete step by step answer:
The emissivity of a non-black body is the fraction between the emissive power of a non- black body to the emissive power of a black body provided both are at the same temperature. The value of emissivity for a non-black body is always less than unity, and it becomes unity if the body is black. Hence, we can conclude that the value of emissivity for any body type is always less than or equal to unity.
Therefore, we can say that emissivity can be defined as the ratio of a real body's emissive power to the emissive power of a black body. Hence, option (A) is correct.

Note: From the concept of law of radiation given by Stefan-Boltzmann, we know that the black body’s emissive power is linearly proportional to the fourth power of absolute temperature of that black body. Stefan-Boltzmann law is derived from Planck's law of thermal radiation which defines the emissive power of the black body for a particular wavelength.