Engineering Questions with Answers - Multiple Choice Questions
Manufacturing Processes – Laser Beam Machining – 2
1 - Question
When coming back to normal state from excited state, electron releases _____
Explanation: On reaching the higher energy level, the electron reaches an unstable energy band. And it comes back to its ground state within a very small time by releasing a photon. This is called spontaneous emission.
2 - Question
In population inversion, no of electrons in _____ are more as compared to numbers of electrons in _____
a) quasi-stable state, ground state
b) meta-stable state, ground state
c) meta-stable state, quasi-stable state
d) mono-stable state, ground state
Explanation: Sometimes such change of energy state puts the electrons in a meta-stable energy band. Instead of coming back to its ground state immediately (within tens of ns) it stays at the elevated energy state for micro to milliseconds. In a material, if more number of electrons can be somehow pumped to the higher meta-stable energy state as compared to the number of atoms at ground state, then it is called “population inversion”.
3 - Question
In laser beam machine, one end of the glass is_____
b) blocked with a 10% reflective mirror
c) blocked with a 75% reflective mirror
d) blocked with a 100% reflective mirror
Explanation: The gas, called lasing medium is enclosed in a cylindrical glass vessel. One end of the glass is blocked with a 100% reflective mirror and the other end is having a partially reflective mirror.
4 - Question
In laser beam machining, electrons are excited by _____
a) high temperature steam
b) flash lamps
c) flash torch
d) cathode ray tube
Explanation: Population inversion can be carried out by exciting the gas atoms or molecules by pumping it with flash lamps. Then stimulated emission would initiate lasing action. Stimulated emission of photons could be in all directions.
5 - Question
The photons emitted in the _____ direction form a laser beam.
Explanation: Most of the stimulated photons, not along the longitudinal direction would be lost and generate waste heat. The photons in the longitudinal direction would form coherent, highly directional, intense laser beam.
6 - Question
How many types of lasers are there?
Explanation: Many materials can be used as the heart of the laser. Depending on the lasing medium lasers are classified as: • Solid-state laser • Gas laser.
7 - Question
How many types of solid state lasers are there?
Explanation: Solid-state lasers are commonly of the following type: • Ruby which is a chromium – alumina alloy having a wavelength of 0.7 μm • Nd-glass lasers having a wavelength of 1.64 μm • Nd-YAG laser having a wavelength of 1.06 μm These solid-state lasers are generally used in material processing
8 - Question
Lasers can be operated in _____ modes
d) only one
Explanation: Lasers can be operated in • continuous mode, or • pulsed mode Typically CO2 gas laser is operated in continuous mode and Nd – YAG laser is operated in pulsed mode.
9 - Question
Helium-Neon is a gas laser.
Explanation: The generally used gas lasers are: • Helium – Neon • Argon • CO2 etc.
10 - Question
Flash tubes used for Nd-YAG laser can be helical or flat.
Explanation: Nd-YAG laser is pumped using a flash tube. Flash tubes can be helical, or they can be flat. Typically the lasing material is at the focal plane of the flash tube. Though helical flash tubes provide better pumping, they are difficult to maintain.