A lens is made of flint glass (refractive index = 1 . 5 ). When the lens is immersed in a liquid of refractive index 1 . 25 , the focal length

Step-by-step solution

Lens-maker's formula is given by 1 f = a μ g - 1 1 R 1 - 1 R 2 … i If the lens is immersed in a liquid of refractive index μ 1 then 1 f 1 = l μ g - 1 1 R 1 - 1 R 2 … i i Here, l μ g is refractive index of glass w.r.t liquid Dividing Eq. (i) by Eq. (ii), we have f 1 f = a μ g - 1 l μ g - 1 ⇒ f 1 f = 1.5 - 1 1.5 1.25 - 1 ⇒ f 1 f = 0.5 × 1.25 0.25 = 2.5 ⇒ f 1 = ( 2.5 ) f Hence, focal length increases by a factor of 2.5.