Question

The radius of currature of each surface of a biconvex lens is 20cm.

(i) Calculate its focal length if of glass = 1.5

(ii) Calculate its focal length when it is immersed in water ( = 1.33)

Answer 1

use formula
f=R/2 (u-1)
where f is focal length , R is radius of curvature and u is refractive index .
now
f=20/2 (1.5-1)=20cm

if lens immersed in water then relative refractive index =1.5/1.33=1.125

now
f=20/2 (1.125-1)=20/(0.25)=80cm