# Engineering Physics Questions and Answers – Population Inversion

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This set of Engineering Physics Multiple Choice Questions & Answers (MCQs) focuses on “Population Inversion”.

1. During Population inversion, which of the following processed is dominant?
a) Stimulated Absorption
b) Stimulated Emission
c) Spontaneous Emission
d) Spontaneous Absorption

Explanation: LASER is a short form of Light Amplification by Stimulated Emission of Radiations. Stimulated Emission is the process by which amplification of radiations takes place. Hence, Laser operation requires stimulated emission to be dominant.

2. The relationship between N1 and N2 for stimulated emission to be dominant is ___________
a) N1 = N2
b) N1 > N2
c) N2 > N1
d) No such relationship

Explanation: In this case, the population of atoms in the exited state is more than that in the ground state. This condition is known as population inversion and in this case, stimulated emission is dominant.

3. The ratio of N2 and N1 is given by ___________
a) e$$\frac{-hv}{kT}$$
b) e$$\frac{hv}{kT}$$
c) e$$\frac{-hv}{T}$$
d) e$$\frac{hv}{T}$$

Explanation: From Maxwell Boltzmann law we know that, N = N0e-E/kT. Hence from there we can say that N2 = N0e-E2/kT and N1 = N0e-E1/kT.
Therefore, N2/N1 = e-(E2 – E1)/kT
= e$$\frac{-hv}{kT}$$.
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4. During pumping, the atoms are exited to ___________
a) Higher Exited States
b) Lower Energy states
c) Meta Stable states
d) Not Excited

Explanation: Pumping is the process in which the atoms from the lower energy states are excited to meta-stable states by some external source so as to create population inversion.

5. At the state when N1 > N2, the intensity of light wave _____________
a) Increases linearly
b) Increases exponentially
c) Decreases linearly
d) Decreases exponentially

Explanation: When N1 > N2, the population of atoms in the ground state is higher than that in the exited state. In this case, absorption is dominant and hence, the intensity of light wave decreases exponentially.

6. To achieve optical amplifications, it is essential to create a non-equilibrium distribution of atoms.
a) True
b) False

Explanation: We know, N2/N1 = e$$\frac{-hv}{kT}$$. Thus, in normal situation, N1 > N2 for E1 < E2. Thus, to achieve lasing action, non-equilibrium distribution of atoms in such a way that the population of the upper energy is greater than that of the lower energy level.

7. What will be the relative population of atoms in a ruby layer that produces a light beam of wavelength 6943 Å at 300 K.
a) 5 X 10-31
b) 6 X 10-31
c) 7 X 10-31
d) 8 X 10-31

Explanation: The relative population of atoms in two states with energy E1 and E2 is given by
N2/N1 = e$$\frac{-(E_2-E_1)}{kT}$$
E2 – E1 = hv
= hc/λ
= 1.79 eV
Therefore, N2/N1 = $$e\frac{-1.79}{kT}$$
= e-69.3
= 8 X 10-31.

8. What will be the relative population of atoms in a ruby layer that produces a light beam of frequency 6 X 1014 Hz at 300 K.
a) e-85
b) e-90
c) e-95
d) e-100

Explanation: We know, N2/N1 = e$$\frac{-hv}{kT}$$
Here, v = 6 X 1014 Hz and T = 300 K
hv/kT = 6.6 X 10-34 X 6 X 1014/ 8.61 X 10-5 X 300
≈ 95
Therefore, N2/N1 = e-95.

9. Between which two layers, population inversion takes place?

a) E1 and E2
b) E1 and E3
c) E2 and E2
d) Neither energy levels

Explanation: Between E2 and E1, there are more electrons in E2 level and it also has higher energy. Hence, population inversion exists between E2 and E1.

10. Which of the following is a four-level laser?
a) ND: YAG
b) Ruby
c) He-Ne
d) Semiconductor laser

Explanation: He-Ne laser is a four-level laser. Whenever an electric discharge is passed through the gas, the helium atoms gets to higher energy states, as the concentration of helium atoms is higher to achieve population inversion.

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