# Engineering Metrology Questions and Answers – Optical Flats

This set of Engineering Metrology Multiple Choice Questions & Answers (MCQs) focuses on “Optical Flats”.

1. How many types of optical flats are present?
a) 2
b) 1
c) 4
d) 3

Explanation: Optical flats are cylindrical in shape and generally have 2 types i.e Type A and Type B. Type A has only one surface flat. Working surface of Type-A is indicated by an arrow. Type B has both surface flat and parallel to each other.

2. Which of the following is used to make optical flats?
a) Toughened glass
b) Glass wool
c) Fused quartz
d) Porous glass

Explanation: Optical flats are generally made from fused quartz or borosilicate glass as they have the least coefficient of linear expansion. This glass is colourless and free from defects like bubbles, internal strains and extraneous matter.

3. What is the coefficient of linear expansion of fused quartz used to make optical flats?
a) 0.6 x 10-6/ degree
b) 0.6 x 10-5/ degree
c) 0.6 x 10-4/ degree
d) 0.6 x 10-3 / degree

Explanation: Fused quartz has 0.6 x 10⁻⁶/degree at standard temperature of 20°C. This low coefficient of linear expansion accounts for its remarkable ability to undergo large, rapid temperature changes without cracking. Borosilicate glass has coefficient of linear expansion not more than 3.6 x 10-6 /degree.

4. To what degree the edges of optical flats are bevelled?
a) 30o
b) 45o
c) 60o
d) 90o

Explanation: Edges of optical flats are generally bevelled to 45o by suitable grinding and polishing to make edges free from any chips. Working surfaces of optical flats are finished by polishing and lapping process and the cylindrical surfaces are finished by grinding.

5. Working surfaces of Type-B optical flats are inclined to each other at which angle?
a) 5 minute
b) 10 minute
c) 15 minute
d) 20 minute

Explanation: The inclination for measuring surfaces of each type of Type-B optical flat is 5 minutes. Optical flats are usually stored in the wooden box to avoid shocks, dust and moisture during storage and transportation.
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6. What is the tolerance on flatness for Type-A optical flats of sizes up to 100 mm?
a) 0.05 cm
b) 0.05 µm
c) 0.05 mm
d) 0.05 m

Explanation: Tolerance on flatness for Type-A opticals flats is 0.05 µm for grade optical flats of size upto 100 mm only. But for the size between 101 mm to 200 mm the tolerance should be 0.10 µm. For grade II, it is 0.01 µm for size upto 100 mm and 0.20 for 101-200 mm.

7. Which of the following is not the dimension of optical flat Type A as per IS : 5440—1969 in mm ?
a) 25 x 10
b) 50 x 30
c) 30 x 10
d) 125 x 30

Explanation: As per IS : 5440-1969 , Optical flats of Type-A have dimensions 25 x 10, 30 x 10, 50 x 15, 75 x 20, 100 x 25, 125 x 30, 160 x 35 ( Diameter in mm x thickness in mm) for both grade I and II.

8. What is the tolerance on thickness for Type-B (Grade I) opticals flats?
a) 0.05 µm
b) 0.15 µm
c) 0.20 µm
d) 0.30 µm

Explanation: Tolerance on flatness, parallelism and thickness for type-B optical flats is 0.05 µm, 0.15 µm and 0.20 µm respectively for grade I and 0.10 µm, 0.20 µm and 0.30 µm for grade II respectively.

9. Which of the following statement is true for optical flats?
Statement 1: Type-A optical flats are tested for parallelism of working surfaces.
Statement 2: Flatness of working surface is tested by comparing with a master flat.
a) T, F
b) F, F
c) T, T
d) F, T

Explanation: Type-A optical flats are used are used to test flatness of working faces and Type-B flats are used to test parallelism of working faces, flatness of working faces and also thickness.

10. Which of the following is the method to test parallelism in Type-B optical flats?
a) Hawking interferometer
b) Enrico interferometer
c) Fizeau interferometer
d) Stephan interferometer

Explanation: Fizeau interferometer is used to test parallelism in optical flats. In this test a light from mercury vapour lamp is focused on eye-piece. Beam splitter is used to reflect the beam which strikes the collimating lens. It collimates the beam and throw it on the surface under test.

11. Type-B optical flat is tested for parallelism. What is the formula for the difference of the thickness of the flat at 2 points on the adjacent fringe? (µ is the refractive index)
a) λ/2*(1/µ) microns
b) λ/4*(µ) microns
c) λ*µ microns
d) λ*(µ/2) microns

Explanation: λ/2*(1/µ) microns, this is the formula for the difference of the thickness of the flat at two points on the adjacent fringes. This is obtained by Fizeau interferometer test. This gives the optical parallelism. This parallelism is also represented as geometrical parallelism as flat is made of material of uniform refractive index.

12. Which of the following is the dimension for Type-B optical flats?
a) 25.0 x 12.000 mm
b) 25.0 x 23.000 mm
c) 26.0 x 24.000 mm
d) 26.0 x 24.250 mm

Explanation: Type-B optical flats have dimensions 25.0 x 12.000, 25.0 x 12.125, 25.0 x 24.250, 25.0 x 12.375, 25.0 x 24.000,25.0 x 24.125, 25.0 x 24.250 and 25.0 x 24.375 respectively.

13. Which of the following is the use of Type-A optical flats?
a) Testing parallelism of measuring devices
b) Testing parallelism of measuring anvils
c) Testing micrometer measuring surfaces
d) Testing the flatness of measuring table

Explanation: Type-A optical flats are used to test the flatness of slip gauges, measuring tables, precision measuring surfaces of flats. Type-B are used to test flatness and parallelism of measuring surfaces of anvils. Micrometer etc.

14. Which of the following is of more important in case of Type-B optical flat?
a) Thickness
b) Diameter
c) Length
d) Area

Explanation: For optical flats of Type-A, grades and optical flats are important that means it is usually designated by these two and for Type-B, thickness and grades are important, therefore it is designated by these two.

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