A convex lens of refractive index 1.5 and focal length 18 cm in air is immersed in water. The change in focal length of the lens will b e_____cm. (Given refractive index of water = 4/3)

Question

G1 Physics · Accepted Answer

Using the lens-maker relation in a medium, 1/f ∝ (μ_lens/μ_medium - 1). In air, μ_lens = 1.5. In water, relative index = 1.5/(4/3) = 9/8, so the refractive power becomes one-fourth of its value in air. Hence f w a t e r f_water f w ​ a t er = 4 f a i r f_air f a ​ i r = 72 cm. Change in focal length = 72 - 18 = 54 cm.