Engineering Physics Multiple Choice Questions on “Introduction and Applications of Laser”.
1. What is the full form of LASER?
a) Light Absorbent and Stimulated Emission of Radiations
b) Light Absorbing Solar Energy Resource
c) Light Amplification by Stimulated Emission of Radiations
d) Light Amplification of Singular Emission of Radiations
Answer: c
Clarification: 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 the meaning of LASER that the light is amplified by stimulating the emission of radiations.
2. In Stimulated Absorption, what is the lifetime of atoms ground state?
a) 1 second
b) 1 minute
c) 1 hour
d) Infinity
Answer: d
Clarification: At the ground state, the atoms are perfectly stable. They are under no excessive force that might lead to become unstable. All the forces are balanced. Thus, as the atom is stable in ground state, its lifetime is infinity.
3. Phonons are __________
a) Quanta of energy
b) Quanta of light waves
c) Quanta of sound waves
d) Quanta of heat
Answer: c
Clarification: Phonons are the quanta of sound waves. When energy is provided, the lattice absorbs energy and gets excited to a higher state. When it de-excites to ground state, it releases radiation in sound-wave region, known as phonons.
4. Which of the following is not a characteristic of LASERS?
a) Monochromatic
b) Coherent
c) Divergent
d) Intense
Answer: c
Clarification: The lasers are highly directional having almost no divergence. The output beam of laser has a well-defined wave front due to which it can be focused on a point.
Lasers are highly intense compared to ordinary light. They are monochromatic and coherent.
5. Laser is used in LIDAR for what purpose?
a) High-Speed Photography
b) Range finder
c) Optical Carrier signal
d) Drilling
Answer: b
Clarification: LIDAR stand for Light Detection and Ranging. Laser is used in LIDAR as range finder. The transit time of transmitted and reflected pulse of laser light is recorded and the distance of the reflecting object is estimated.
6. The output of a laser has pulse duration of 30 ms and average output power of 1 W per pulse. How much energy is released per pulse if wavelength is 6600 Å?
a) 0.001 J
b) 0.002 J
c) 0.003 J
d) 0.004 J
Answer: c
Clarification: As we know, Energy = Power X Time
= 1 W X 30 X 10-3s
= 0.003 J.
7. Laser light from a 2mW source of aperture diameter 1.5 cm and wavelength 5000 Å is focused by a lens of focal length 20 cm. The intensity of the image is __________
a) 1.57 X 106 Wm-2
b) 2.57 X 106 Wm-2
c) 3.57 X 106 Wm-2
d) 4.57 X 106 Wm-2
Answer: c
Clarification: Area of the image = (frac{pilambda^2f^2}{a^2})
λ = 5000 Å = 5.0 X 10-7m, f = 0.2 m
a = 1.5/2 cm = 0.75 X 10-2 m
Putting in the formula we get, Are = 5.6 X 10-10 m2
Intensity = Power/Area
= 2 X 10-3W/ 5.6 X 10-10 m2
= 3.57 X 106 Wm-2.
8. For an ordinary light source, the coherence time t = 10-10 s. The degree of Monochromaticity for a wavelength of 6000 Å is ___________
a) 0.1 X 10-4
b) 0.2 X 10-4
c) 0.3 X 10-4
d) 0.4 X 10-4
Answer: b
Clarification: Coherence time, t = 10-10 s
Therefore, Δv = 1/t = 1010 Hz
Now, λo = 5000 Å, vo = 5.0 X 1014 Hz
Monochromaticity = (frac{delta v}{v_0})
= 0.2 X 10-4.
9. Lasers are used for welding of wires because they can be focused onto a fine spot.
a) True
b) False
Answer: a
Clarification: Laser beams are highly directional with almost no convergence. Thus, they can be focused onto a fine spot with ease. Due to this, they are used in welding of fine wires, contacts in miniature assemblies, drilling holes etc.
10. Where is ND: YAG most commonly used?
a) Cosmetic Surgery
b) Welding
c) Photography
d) Optical Communications
Answer: a
Clarification: ND: YAG is most commonly used for cosmetic energy because it has the property of maximum energy absorption by the target (hair or lesion) with minimum absorption by the surrounding skin structures.
11. The information carrying capacity of laser is enormous due its large _________
a) Coherence
b) Bandwidth
c) Directionality
d) Intensity
Answer: b
Clarification: Laser has a large bandwidth. The rate at which the information can be transmitted is proportional to bandwidth and the bandwidth is proportional to carrier frequency. Because of these properties, Laser is widely used as optical carrier signal.
12. Which characteristic of LASER allows it to be used in holography?
a) Coherency
b) Directionality
c) Intensity
d) Monochromaticity
Answer: a
Clarification: The production of an image in a hologram takes place via a process called reconstruction. In this process, the image is “reconstructed” in the form of a hologram. This reconstruction if possible, via LASER as they are highly coherent.
13. What is the region enclosed by the optical cavity called?
a) Optical Region
b) Optical System
c) Optical box
d) Optical Resonator
Answer: d
Clarification: The optical cavity resembles an oscillator as it provides feedback of the photons by reflection, at the mirrors. Therefore, the area enclosed inside the optical cavity is called optical resonator.
14. The following graph is pictorial representation of _________
a) Spontaneous emission
b) Spontaneous Absorption
c) Stimulated emission
d) Stimulated Absorption
Answer: c
Clarification: The diagram shows that when a photon from the incident radiation, having energy E2 – E1, interacts with the atom in excited state, the atom comes down to the ground state with the emission of a photon with same frequency, phase and direction of propagation.