Question

Focal length of a convex lense in air is `10cm`. Find its focal
length in water. Given that `mu_g=3//2` and `mu_w=4//3`.

Answer 1

Using the relations,
`1/(f_(air))=(mu_(g)-1)(1/R_(1)-1/R_(2))`
and `1/(f_("water"))=((mu_(g))/(mu_(w))-1)(1/R_(1)-1/R_(2))`
Dividing Eq. (i) by (ii) we get
`(f_("water"))/(f_(air))=((mu_(g)-1))/((mu_(g)//mu_(w)-1))`
Substituting the values we have
`f_("water")=((3//2-1))/(((3//2)/(4//3)-1))f_(air)=4f_(air)=4xx10=40 cm`