Question

Statement 1 : Emissivity of any body $\left(e\right)$ is equal to its absorptivity $\left(a\right)$ at thermal equilibrium.
Statement 2 : The ratio of emissive power to the absorptive power is different for different bodies at a given temperature.

• A. Statement 1 is true, statement 2 is false
• B. Statement 1 is false, statement 2 is true
• C. Both Statement 1 and statement 2 are true
• D. Both Statement 1 and statement 2 are false

Answer 1

The correct option is A Statement 1 is true, statement 2 is false

According to Kirchoff's law,
The ratio of emissive power $\left(E\right)$ to the absorptive power $\left(a\right)$ is same for all bodies at a given temperature and is equal to emissive power of a black body.
$\frac{E_1}{a_1}=\frac{E_2}{a_2}=\frac{E_{bb}}{a_{bb}}=E_{bb}=$constant
$[$For black body, $a_{bb}=1$$]$

Also, at thermal equilibrium radiation emitted by a body is equals to the radiation absorbed by the body. Hence, emissivity of any body $\left(e\right)$ is equal to its absorptivity $\left(a\right)$ at thermal equilibrium.

Therefore, option $\left(a\right)$ is the correct answer.