Question

A double convex lens (μ=1.5) has its focal length 10 cm. What will be its focal length if it is immersed in liquid of (μ1=43)

Answer 1

The only parameter that changes is n0, the refractive index of surrounding media.

P(lens) is the inverse of the focal length.

Since lens is immersed in a medium of higher refractive index, its lens strength is expected to be lower. This means it will focus loosely to a distant location.

When in air : P1 = (n-1) (X), where X = (1/R1- 1/R2)

When in water : P2 = (n-1.33) (X)/ 1.33

Solving them we get, P1/P2 = 1.33 (n-1) / (n-1.33)

P1 = 1/20 cm = 5 diopters

Assuming n = 1.5 (glass lens), P2 = 1.28 (approx).

Focal length = 78 cm