Question

For an object other than a black body, it’s emissivity, e is _______________

(a) 1

(b) 0 < e < 1

(c) e > 1

(d) e = 0

This question was posed to me in final exam.

The above asked question is from Black Body Radiation topic in portion Quantum Physics of Engineering Physics I

Answer 1

The correct choice is (b) 0 < e < 1

Easy explanation: Emissivity is the ratio of emissive power of any object and the emissive power of the black body having the same temperature and surface area as the object. Thus, for a black body, it is equal to 1. For any other object, it is less than 1.