Question

1.6 is a refractive index of plano–convex lens, then the radius of curvature of the curved surface is 60 cm. The focal length of the lens is __________ cm 

(A) 50 (B) 100 (C) – 50 (D) – 100

Answer 1

The correct option is (B) 100.

Explanation:

For a plano convex lens, f = [(R) / (η –1)] 

η = refractive index = 1.6 

R = radius of curvature = 60 cm 

∴ f = [(60) / (1.6 – 1)] = [(60) / (0.6)] = 100 cm.