Question

the radius of currature of each surface of a biconvex  lens is 20 cm 

calculate its focal length if u of glass (1.5)

calculate it's focal length what it is immersed in water( u=1.33)

Answer 1

use formula
f=R/2 (u-1)
=20/2(1.5-1)=20cm
when lens immersed in water
then relative refractive index=1.5/1.33=4.5/4=1.125
now focal length
f=R/2 (1.125-1)
=20/2 (0.125)=20/0.25=80cm