Question

How many diffraction gratings are used by interferometer in grazing incident technique with laser beam system?

(a) 1

(b) 2

(c) 3

(d) 4

This question was posed to me in an international level competition.

Question is from Grazing Incident Technique to Evaluate Surface Flatness topic in section Machine Vision Systems of Engineering Metrology

Answer 1

Correct answer is (b) 2

To elaborate: The interferometer uses 2 linear diffraction gratings. These gratings are used to split and recombine the light into reference and test beams. The diffracted light of first order from the first grating reflects off at a grazing angle and it is the test beam. The zero order diffracted light is reference beam and passes parallel to the surface.