# Engineering Metrology Questions and Answers – Interferometry Applied to Flatness Testing

This set of Engineering Metrology Questions and Answers for Experienced people focuses on “Interferometry Applied to Flatness Testing”.

1. Which of the following is the incorrect condition for a perfectly flat surface when tested for interferometry?
a) Monochromatic light is used
b) Viewing angle should be greater than 0o
c) Optical flats are important in light wave measurement
d) For perfect flat surface alternate light and dark bands are observed

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

2. If the angle at which bands are viewed is 30o in flatness testing with interferometry, what is the possible error?
a) 0%
b) 15%
c) 30%
d) 45%

Explanation: If the viewing angle is 30 degrees then three fringes may be seen as 2.5 that is it will cause error of 15 %. If the viewing angle is 45 degrees then three fringes may be seen as 1.75 that is it will cause nearly 40% error.

3. What is the range of the diameter of optical flats?
a) 10 to 50 mm
b) 50 to 100 mm
c) 100 to 200 mm
d) 25 to 300 mm

Explanation: Optical flats are the circular pieces of optical glass with a high degree of finish on the working surfaces. Its two plane faces are flat and parallel to each other. Optical flats varies from the size of 25 to 300 mm.

4. In flatness testing with interferometry, if surfaces are perfectly wrung. Then which option is correct regarding fringe pattern?
a) No fringe pattern
b) Fringes are closer
c) Fringes are at great distance
d) Indistinguishable pattern

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

5. Which of the following is true for quartz flats?
a) Have 200 to 400 % more wearing quality than optical flats
b) Have 10 % more wearing quality than optical flats
c) Have 10 % less wearing quality than optical flats
d) Have 200 to 400 % less wearing quality than optical flats

Explanation: Quartz flats have more wearing qualities than optical flats. It is 200 to 400 percent more than optical flats. Working surfaces are finished to a high optical degree of flatness.
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6. Which of the following option is incorrect for flatness testing by interferometry?
a) For checking concave or convex surface optical flat is pressed lightly
b) If angle between optical flat and work surface reduces, then fringe spacing decreases
c) For perfectly flat surface condition of complete interference satisfies
d) Only one point contact between optical flat and surface is possible in some cases

Explanation: If ‘θ’ that is the angle between optical flat and work surface whose flatness to be measured reduces then the fringe space increases and if ‘θ’ increases then fringes brought more closer that is fringe spacing decreases.

7. If the angle between optical flat and surface to be tested is very small, then what is the difference of separation between optical flat and surface between two similar adjacent fringes?
a) λ
b) λ/2
c) λ/3
d) 3λ/2

Explanation: Change in separation between surface and optical flat between 2 adjacent and similar fringes is equal to (3λ/4 – λ/4) = λ/2.

8. What is the pattern of fringes, if an optical flat is resting on any hill?
a) Central band dark
b) Concentric dark and light band fringes
c) Straight dark fringes
d) Central band may be light or black

Explanation: If 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 central bright fringe.

9. Which of the following option is correct for the given statements about interferometry applied to flatness testing?
Statement 1: If the angle between optical flat and surface is too large then there will be largely spaced fringe pattern is observed.
Statement 2: Quartz flats are very sensitive to heat.
a) T, T
b) F, T
c) T, F
d) F, F

Explanation: If ‘θ’ angle between optical flat and surface whose flatness to be measured is too large then fringes will be too closely placed as to be indistinguishable. Quartz have minimum expansion due to heat.

10. Which of the following option is true for optical flats used in flatness testing?
a) Type A optical flats has only one surface flat
b) Optical flats are better than quartz flats
c) White light source is very important in measurement by light wave interference
d) Working surfaces of optical flats are cylindrical

Explanation: Type A optical flats have only one flat surface. Working surface in such type of optical flats is indicated by an arrow head on the cylindrical surface. This arrow head is pointing towards the working surface.

11. To distinguish hill and valley one surface of optical flat is lightly pressed. Which of the following option is for the valley?
a) Number of fringes increases
b) Fringes move apart
c) Fringes brought closer
d) Centre of fringes is displaced

Explanation: In case of hill or convex surface, centre of fringes is displaced and fringe space decreases. But if by light pressure number of fringes decreased and fringes move apart it is a valley or concave surface.

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