Question

A light of wavelength 6000Å incident on a convex lens of its refractive

index 1.5 and is placed in a liquid of same refractive index as the lens,

since the light passes parallel to principal axis after refraction. The

wavelength of refractive light will be

(a) 4000 Å

(b) 3000 Å

(c) 6000 Å

(d) 7000 Å
​

Answer 1

answer:

c

Explanation:

Here, λ=6000Å=6×10−7m,

μ=1.5,r=60∘,t=?

For the film to appear dark in the reflected system, 2μtcosr=nλ

For smallest thickness of film, n=1

∴2μtcosr=λ ,brgt t=λ2μcosr=6×10−72×1.5cos60∘=4×10−7m