Question

The focal length of a convex lens made of glass is 20 cm. What will be its new focal length when placed in a medium of refractive index 1.25 ?

Answer 1

$\frac{1}{f}=(\frac{{\mu }_1}{{\mu }_2}-1)(\frac{1}{r_1}-\frac{1}{r_2})=\frac{1}{20}$

now,

$\frac{1}{f‘}=(\frac{1}{1.25}-1)(\frac{1}{r_1}-\frac{1}{r_2})=\frac{-1}{5}\ast \frac{1}{20}$

$f‘=(-5)f$