Question

A convex lens has 10cm focal length in air what is its focal length in water

Answer 1

Air=10cm.
Water=?.

find v and u by formula :1/f=1/v-1/u.

then find it in water.

Answer 2

If n is refractive index then

1/f=(n-1)(1/R’-1/R”)

let 1/R’-1/R”)=k.

then 1/fa=[n(ag) -1] k

if n(ag) = 3/2, n(Aw) =4/3

then n(wg) =n(ag) /n(aw)

=(3/2)*(3/4)=9/8

if fa=10cm

here fa is focal length of glass lens in air and

fw is focal length of glass lens in water then

1/fw=[n(wg)-1]k

1/fw=(9/8-1)k and

1/10=(3/2-1)k

dividing

10/fw=(9/8-1)/(3/2-1)

=(1/8)/(1/2)

10/fw=1/4

fw=10*4

fw=40cm. Answer