A biconvex lens of refractive index 1 . 5 has a focal length of 20 cm in air. Its focal length when immersed in a liquid of refractive index 1 . 6 will be:

Step-by-step solution

Given, the refractive index of the material of the lens is μ 1 = 1 . 5 , the refractive index of the medium is μ m = 1 . 6 and the focal length of the lens in air is f a = 20 cm . In air: 1 f a = μ 1 - 1 2 R In liquid: 1 f m = μ 1 μ m - 1 2 R The focal length as measured in air and that as measured inside the liquid medium are related by, f m f a = μ 1 - 1 μ m μ 1 - μ m . . . 1 Equation (1) implies that f m 20 = ( 1 . 5 - 1 ) 1 . 6 ( 1 . 5 - 1 . 6 ) ⇒ f m = - 160 cm