Question

Good emitters are good absorbers is a statement concluded from

• A. Newtons law of cooling
• B. Stefans law of radiation
• C. Provosts theory
• D. Kirchhoffs law

Answer 1

Answer: (D)

According to Kirchhoffs law the ratio of emissive power to absorptive power is same for all surfaces at the same temperature and is equal to the emissive power of a perfectly black body at that temperature.
Hence, $\frac{e_1}{a_1}=\frac{e_2}{a_2}=\dots {\left[\frac{E}{A}\right]}_{\text{perfectly black body }}$
Now, since ${\left(E_{\lambda }\right)}_{\text{black }}$ is constant at a given temperature, this implies that good absorber is a good emitter (or radiator).