Interferometer MCQ || Interferometer Questions and Answers

1.   ______ works by merging two or more sources of light to create an interference pattern, which can be measured and analyzed.

  1. Interferometer
  2. Power Meter
  3. Optical Meter
  4. None of the above

Answer.1. Interferometer


Interferometers are investigative tools used in many fields of science and engineering. They are called interferometers because they work by merging two or more sources of light to create an interference pattern, which can be measured and analyzed; hence ‘Interfere-o-meter’, or interferometer.


2. What does N, P and L mean in the N.P.L. Gauge interferometer?

  1. Nikon pulsed laser
  2. Nuclear plasma laboratory
  3. National Physics Laboratory
  4. Nuclear physics laboratory

Answer.3. National Physics Laboratory


N, P and L in N.P.L. Gauge interferometer stands for National Physics Laboratory.

  • Its headquarter is in New Delhi. It was established on January 4, 1947.
  • On March 17 it established an atmospheric monitoring station on the campus of the Institute of Himalayan Bioresource Technology (IHBT) at Palampur, Himachal Pradesh.
  • It is situated at an altitude of 1391m for generating the base data for atmospheric trace species & properties to serve as a reference for comparison of the polluted atmosphere in India.


3. NPL Gauge Block Interferometer measure the length of gauge blocks upto _____

  1. 1000 mm
  2. 2000 mm
  3. 500 mm
  4. 300 mm

Answer.3. 300 mm


The NPL Gauge Block Interferometer was designed to measure the length of gauge blocks, length bars and Hoke gauges up to 300 mm in length. Up to 12 gauges can be wrung to a platen which is then placed in the instrument and measured after a suitable thermal settling period.


4. Which type of fringe is formed, if a path difference of λ/2 is observed in an interference pattern?

  1. Dark fringe
  2. Bright fringe
  3. Both bright and dark fringe overlap
  4. Unpredictable

Answer.1. Dark fringe


The dark fringes are formed due to destructive interference. Interference is said to be destructive if the resultant amplitude and hence the resultant intensity is zero. The condition for destructive interference is that the phase difference between the two waves must be an odd multiple of π and the path difference between the two waves is equal to an odd multiple of λ/2.


5. At which angle does a glass plate reflector set in N.P.L. interferometer?

  1. 30°
  2. 45°
  3. 60°
  4. 90°

Answer.2. 45°


The fringes obtained can be viewed directly above the means of a thick glass plate semi-reflector set at 45° to the optical axis. The various results can be studied for comparison.


6. Which of the following is the incorrect condition for a perfectly flat surface when tested for interferometry?

  1. Monochromatic light is used
  2. Viewing angle should be greater than 0°
  3. Optical flats are important in light wave measurement
  4. For perfect flat surface alternate light and dark bands are observed

Answer.2. 45°


Optical flats are devices made of lapped quartz that are used to read light-wave interference patterns to measure the degree of flatness of a surface. The surface must have a finish of 2 µinch or better to successfully use an optical flat to check it. In addition, the wavelength of the light reflecting through the optical flat must be known.

The reason the optical flat must have a fine finish is straightforward: When an optical flat is laid on the surface of the part to be finished, it has a film of air under it; that air is important to permit the reflection of the fringe lines used to measure the surface.

Note:- Angle at which bands are viewed is called the viewing angle. The viewing angle should be 0o that is a line of sight should be at 90 degrees to the reference surface of optical flats. As this angle increases it will cause an error.


7. In flatness testing with interferometry, if surfaces are perfectly wrung. Then which option is correct regarding fringe pattern?

  1. No fringe pattern
  2. Fringes are closer
  3. Fringes are a great distance
  4. Indistinguishable pattern

Answer.1. No fringe pattern


If the surface between optical flat and work surface are perfectly wrung together then there will be no air gap between the surfaces and the fringe pattern will be not observable.


8. A Michelson Interferometer has two equal arms. A mercury light of wavelength 546 nm is used for the interferometer and stable fringes are found. One of the arms is moved by 1.5 μm. The number of fringes that cross the observing field is __________

  1. 10
  2. 5
  3. 15
  4. 20

Answer.2. 5


When mirror M2 moves a distance ‘d’, then the relation between wavelength λ and number of fringes is given by,

d = mλ/2


m = d2/λ


λ = 564 nm = 546 × 10-9 m

d = 1.5 μm = 1.5 × 10-6 m

m = 546 × 10-9 × 2/1.5 × 10-6

m = 3000/546 = 5.49


9. What is the pattern of fringes, if an optical flat is resting on any hill?

  1. Central band dark
  2. Concentric dark and light band fringes
  3. Straight dark fringes
  4. Central band may be light or black

Answer.2.Concentric dark and light band fringes


Concentric dark and light band fringes are the pattern of fringes if an optical flat is resting on any hill. If the surface has a large number of valleys and hills. Generally, optical flat makes contact with hills. Then it will just behave as if it was placed on the convex surface. There will be concentric bright and dark fringes with a central bright fringe.


10. To distinguish hill and valley one surface of the optical flat is lightly pressed. Which of the following option is for the valley?

  1. Number of fringes increases
  2. Fringes move apart
  3. Fringes brought closer
  4. The Centre of fringes is displaced

Answer.2. Fringes move apart


The distance between any two consecutive bright fringes or two consecutive dark fringes is called fringe spacing. Fringe spacing or thickness of a dark fringe or a bright fringe is equal. In case of hill or convex surface, center of fringes is displaced and fringe space decreases. But if by light pressure number of fringes decreases and fringes move apart it is a valley or concave surface.

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