# Engineering Physics Questions and Answers – Laser

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

1. Which of the following is a unique property of laser?
a) Directional
b) Speed
c) Coherence
d) Wavelength

Explanation: Coherence is an important characteristic of laser beam because in laser beams, the wave trains of the same frequency are in phase/ Due to high coherence it results in extremely high power.

2. Which of the following is an example of optical pumping?
a) Ruby laser
b) Helium-Neon laser
c) Semiconductor laser
d) Dye laser

Explanation: The atoms of Ruby are excited with the help of photons emitted by an external optical source. The atoms absorb energy from photos and raises to excited state. Therefore Ruby laser is an example of optical pumping.

3. When laser light is focussed on a particular area for a long time, then that particular area alone will be heated.
a) True
b) False

Explanation: Laser beam has very high intensity, directional properties and coherence. When it is focussed on a particular area for a long time, then the area alone will be heated and the other area will remain as such. This is called thermal effect.

4. Calculate the wavelength of radiation emitted by an LED made up of a semiconducting material with band gap energy 2.8eV.
a) 2.8 Å
b) 4.3308 Å
c) 5548.4 Å
d) 4430.8 Å

Explanation: E = hc/ʎ
Therefore, ʎ = hc/E
ʎ = 4430.8 Å.

5. Calculate the number of photons, from green light of mercury (ʎ = 4961 Å), required to do one joule of work.
a) 4524.2×1018/m3
b) 2.4961×1018/m3
c) 2.4961/m3
d) 2.4961/m

Explanation: E = hc/ʎ
E = 4.006×10-19 Joules
Number of photons required = (1 Joule)/(4.006×10-19)
N = 2.4961×1018/m3.
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6. Which of the following is ideal for the generation of laser pulse?
a) Ruby laser
b) Carbon dioxide laser
c) Helium neon laser
d) Nd-YAG laser

Explanation: Since Nd YAG laser has a higher thermal conductivity than other solid state lasers, it lends itself for the generation of laser pulses at a higher pulse repetition rate or a quasi continuous wave operation.

7. What is the need to achieve population inversion?
a) To excite most of the atoms
b) To bring most of the atoms to ground state
c) To achieve stable condition
d) To reduce the time of production of laser

Explanation: When population inversion is achieved, the majority of atoms are in the excited state. This causes amplification of the incident beam by stimulated emission. Thus the laser beam is produced.

8. Laser is called as a non-material knife.
a) False
b) True

Explanation: In laser surgery, without knife, bloodless operation, cutting tissues etc can be made, hence laser is called non-material knife.

9. DVD uses the laser.
a) True
b) False

Explanation: A DVD player contains a laser. By moving the lens longitudinally, different depths can be reached in the disc. In order to make room for a lot of information on every disc, the beam has to be focused on as small an area as possible. This cannot be done with any other light source.

10. Which of the following is used in atomic clocks?
a) Laser
b) Quartz
c) Maser
d) Helium

Explanation: Before laser maser was used. It stood for microwave amplification by stimulated emission of radiation. This was based on Albert Einstein’s principle of stimulated emission. It was used in the atomic clock.

11. Which of the following can be used in the vibrational analysis of structure?
a) Maser
b) Quarts
c) Electrical waves
d) Laser

Explanation: Laser can be used in the vibrational analysis of the structure. This is because when a structure under test begins to vibrate a distinctive pattern begins to emerge.

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