# The Black Body and Kirchoff's Law - 2 - MCQs

## The Black Body and Kirchoff's Law – 2 – MCQs

1. What does the Kirchoff's identity state?

a. the emissivity and the absorptivity of the black body are same and always less than 1
b. the emissivity and the absorptivity of the black body are same and always more than 1
c. the emissivity and the absorptivity of the black body are same and always equal to 1
d. none of the above

ANSWER: c. the emissivity and the absorptivity of the black body are same and always equal to 1

2. Stefan-Boltzmann law states that the energy radiated per unit area per unit time by the black body is

a. directly proportional to the square of the temperature of the black body
b. inversely proportional to the square of the temperature of the black body
c. directly proportional to the forth power of the temperature of the black body
d. inversely proportional to the fourth power of the temperature of the black body

ANSWER: c. directly proportional to the forth power of the temperature of the black body

3. What is the SI unit for Stefan-Boltzmann constant (σ)?

a. W / m K4
b. W / m2 K4
c. W / K4
d. W K4 / m2

ANSWER: b. W / m2 K4

4. Which among the following sentences are correct for Stefan-Boltzmann law?

(1) The Stefan-Boltzmann law gives the total emissive power of a black body.
(2) The Stefan-Boltzmann law is based on quantum theory
(3) The Stefan-Boltzmann law does not provide distribution of energy among different wavelengths

a. sentences (1) and (2) are correct
b. sentences (2) and (3) are correct
c. sentences (1) and (3) are correct
d. all of the sentences (1), (2) and (3) are correct

ANSWER: c. sentences (1) and (3) are correct

5. The relation derived for the total emissive power of the black body as a function of wavelength and based of quantum theory is known as

a. Kirchoff's law
b. Stefan-Boltzmann law
c. Planck's law
d. none of the above

ANSWER: c. Planck's law

6. Wien's displacement law gives the relationship between

a. the wavelength of radiation at which total emissive power is maximum and the temperature
b. the wavelength of radiation at which total emissive power is minimum and the temperature
c. the wavelength of radiation at which total emissive power does not change and the temperature
d. none of the above