Question

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

The question was posed to me at a job interview.

My doubt is from Interferometry Applied to Flatness Testing in portion Measurement by Light-Wave Interference of Engineering Metrology

Answer 1

Right choice is (b) Fringes move apart

To elaborate: 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.