Kirchoff's Law

Q. 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.

1. Statement 1 is true, statement 2 is false
2. Statement 1 is false, statement 2 is true
3. Both Statement 1 and statement 2 are true
4. Both Statement 1 and statement 2 are false

Q.

Which one of the following statements is not true about thermal radiations?

1. All bodies emit thermal radiations at all temperatures

2. Thermal radiations are electromagnetic waves

3. Thermal radiations are not reflected from a mirror

4. Thermal radiations travel in free space with a velocity of $3\times {10}^{8}m{s}^{-1}$

Q. The emissive power of a black body at $T=300\text{K}$ is $100{\text{W/m}}^2$. Consider a body $B$ of area $A=10{\text{m}}^2$, coefficient of reflectivity $r=0.3$ and coefficient of transmission $t=0.5$. Its temperature is $300\text{K}$. Then choose the correct statement.

1. The emissive power of $B$ is $20{\text{W/m}}^2$.
2. The emissive power of $B$ is $200{\text{W/m}}^2$.
3. The power emitted by $B$ is $100\text{W}$.
4. The emissivity of $B$ is $0.2$.

Q. A person with black clothes experiencing more heat and cold as compared to a person with white clothes is an application of

1. Kirchhoff’s law
2. Lambert’s law
3. Planck’s law