This set of Engineering Physics Multiple Choice Questions & Answers (MCQs) focuses on “Ruby Laser”.
1. Which of the following is a three-level laser?
a) ND: YAG
b) Ruby
c) He-Ne
d) Semiconductor laser
View Answer
Explanation: Ruby laser is a three-level laser. Ruby crystal is a crystal of Al2O3 with some Al3+ ions replaced by Cr3+ ions. The energy levels of Cr3+ are responsible for lasing action.
2. The lifetime of meta-stable state in a Ruby laser is ___________
a) 10-8s
b) 10-6s
c) 10-3s
d) 10-2s
View Answer
Explanation: The second energy level in a Ruby laser is the meta-stable state. It’s lifetime is 10-3 s. It is between this state and the ground state that the lasing action takes place.
3. The ends of the ruby rod works as __________
a) Pumping source
b) Active medium
c) Cavity mirrors
d) Energy levels
View Answer
Explanation: In the ruby lase, one end of the ruby rod is completely polished while the other end is made partially reflecting. Hence, the two ends work as the cavity mirrors.
4. The pumping mechanism used in Ruby rod is __________
a) Optical Pumping
b) Electrical Excitation
c) Chemical pumping
d) Thermal pumping
View Answer
Explanation: The Ruby rod is placed inside a xenon flash lamp so as to provide sufficient amount of light to cause excitation of atoms. This kind of pumping is called optical pumping.
5. Which material is used for cooling of the ruby rod for efficient continuous operation?
a) CFC
b) Liquid helium
c) Liquid oxygen
d) Liquid Nitrogen
View Answer
Explanation: A large amount of energy is dissipated in the ruby rod. Thus, it has to be cooled for efficient continuous operation. For this purpose, liquid nitrogen is used.
6. The laser beam is emitted in the form of __________
a) Fluctuating radiations
b) Continuous spectrum
c) Pulsed output
d) Exponentially decreasing intensity
View Answer
Explanation: The laser output observed in a Ruby laser is pulsed. This is so, as the flash lamp operation is pulsed. There are spikes in the ruby laser output, lasting a few milliseconds.
7. The following graph represents the output for which type of LASER?
a) He-Ne Laser
b) Ruby Laser
c) ND: YAG laser
d) Semiconductor Diode laser
View Answer
Explanation: Let there be three energy levels E1, E2 and E3 with E3 > E2 > E1.
Now, when population inversion is achieved between E1 and E2, simulates emission starts and coherent laser light is produced with a sharp peak at 6943 A. It depopulates the level E2 at faster rate than pump rate, stopping laser action momentarily. Before the output falls to zero, the process starts repeating. Due to this, the output consists a series of spikes.
8. For a Ruby laser, the coherence time t = 10-10 s. The degree of Monochromaticity is __________
a) 0.13 X 10-4
b) 0.24 X 10-4
c) 0.35 X 10-4
d) 0.47 X 10-4
View Answer
Explanation: Coherence time, t = 10-10 s
Therefore, Δv = 1/t = 1010 Hz
Now, λo = 6940 Å, vo = 7.2 X 1014 Hz
Monochromaticity = \(\frac{Δv}{v_0}\)
= 0.13 X 10-4.
9. 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
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.
10. What is the wavelength of the emitted laser in a Ruby laser?
a) 694 nm
b) 650 nm
c) 780 nm
d) 754 nm
View Answer
Explanation: Ruby laser emits visible light. It is a three-level laser. The emitted radiations have a wavelength of 694.3 nm, which represents the red color. The output observed in a Ruby laser is spiked.
11. The energy levels of which ion/atom/molecule are responsible for lasing action?
a) Al3+
b) Cr3+
c) Al2O3
d) Cr2O3
View Answer
Explanation: The energy levels of Cr3+ are responsible for the lasing action. It is the three energy levels of Cr3+ ion, that the population inversion takes place and light is emitted via stimulated emission.
Sanfoundry Global Education & Learning Series – Engineering Physics.
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