Question

the focal length of a convex lens made of the glass is 20 cm. what will be it's new focal length when placed in a medium of refractive index 2.25?

Answer 1

in vaccum focal length =20cm
hence f=R/2
R=2f=40cm
when lens immersed in a medium which refractive index 2.25 then,
f=R/2 (u-1)
=40/2 (2.25-1)=40/2 (1.25)=40/2.5=16cm

Answer 2

in vaccum focal length =20cm

hence f=R/2

R=2f=40cm

when lens immersed in a medium which refractive index 2.25 then,

f=R/2 (u-1)

=40/2 (2.25-1)

=(1.25)=40/2.5

=16cm