Engineering Physics Questions and Answers – Population Inversion

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
View Answer

Answer: b
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
View Answer

Answer: c
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\(\frac{hv}{T}\)
View Answer

Answer: a
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\(\frac{-hv}{kT}\).

4. During pumping, the atoms are exited to ___________
a) Higher Exited States
b) Lower Energy states
c) Meta Stable states
d) Not Excited
View Answer

Answer: c
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
View Answer

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

Answer: a
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
View Answer

Answer: d
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
View Answer

Answer: c
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?
The population inversion takes place E1 & E2 in E2 level between two layers
a) E1 and E2
b) E1 and E3
c) E2 and E2
d) Neither energy levels
View Answer

Answer: a
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 not a four-level laser?
a) CO2
b) Ruby
c) He-Ne
d) Er:YAG
View Answer

Answer: b
Explanation: Ruby laser is a 3-level laser whereas all others are 4-level.

Sanfoundry Global Education & Learning Series – Engineering Physics.

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Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He lives in Bangalore, and focuses on development of Linux Kernel, SAN Technologies, Advanced C, Data Structures & Alogrithms. Stay connected with him at LinkedIn.

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