The radii of curvature of a lens are +20 cm and +30 cm. The material of the lens has a refracting index 1.6. Find the focal length of the lens

(a) If it is placed in air, and

(b) If it is placed in water (μ = 1.33).

Radii of curvature of a lens are:

Refractive index of material == 1.6

Refractive index of water = = 1.33

(a) When lens is placed in air

By using the lens makers’ formula;

We get;

(b) If it is placed in water (μ = 1.33).

By using the lens makers’ formula;
we get;