Kirchhoff's Law of Radiation

 At temperature $T$ , the emissive power and absorption power of a body for certain wavelength are $e_{\lambda }$  and $a_{\lambda }$  respectively, then

At a certain temperature for given wavelength, the ratio of emissive power of a body to emissive power of the black body in the same circumstances is known as

If $E$ Represents emissivity of a substance ,$E_b$ Represents emissivity of black body and $a$ Represents coefficient of absorption, then correct equation out of the following is

Emissivity of perfectly black body is