Question

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

I have been asked this question in class test.

The question is from Interferometry Applied to Flatness Testing topic in section Measurement by Light-Wave Interference of Engineering Metrology

Answer 1

Correct choice is (b) Concentric dark and light band fringes

The best I can explain: 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.