Question

Explain why the central fringe in Newton’s rings is dark in the case of reflected system.

Answer 1

At the center the thickness of the air film formed between lens and glass plate is zero. Therefore , at the center the geometrical path difference between the rays incident and reflected from glass plate is zero,but due to reflection a path difference of (lambda/2) is introduced. This path difference gives distructive interference at the center and hence center is dark.