Question

A biconvex thin lens of refractive index μ₂ =1.4  has a radii of curvature R₁ = 20 cm and R₂ = 60 cm, μ₁ = 1.5 and  μ₃ =1.2 . Focal lengths are f₁ and f₂ in both the media

(a) The lens behaves as a concave lens

(b)  f₁ = f₂

(c)   f₁ ≠ f₂

(d) The lens behaves as a convex lens

Answer 1

Correct option  (c)   f₁ ≠ f₂

Explanation :

1/f₁ = n₂ - n₁/R₁ = (0.1)/(20 cm)

1/f₂ = n₃ - n₂/R₂ = (0.2)/(60 cm)

So overall focal length is (­ve)

Hence concave lens.

f₁ ≠ f₂