Question

The spherical surface of a piano-convex lens comes into contact with a glass plate. The space between the lens and the plate is filled up with carbon dioxide. The refractive indices of the lens, carbon dioxide, and the plate are equal to n₁ = 1.50, n₂ = 1.63, and n₃ = 1.70 respectively. The curvature radius of the spherical surface of the lens is equal to R = 100 cm. Determine the radius of the fifth dark Newton's ring in reflected light with wavelength λ = 0.50 μm.

Answer 1

The condition for minima are

(There occur phase changes at both surfaces on reflection, hence minima when path difference is half integer multiple of λ).