Question

A concave lens with unequal radii of curvature made of glass $({\mu }_g=1.5)$ has a focal length of $40cm$. If it is immersed in a liquid of refractive index ${\mu }_1=2$, then

• A. It behaves like a convex lens of $80cm$ focal length
• B. It behaves like a convex lens of $20cm$ focal length
• C. Its focal length becomes $60cm$
• D. Nothing can be said

Answer 1

The correct option is A It behaves like a convex lens of $80cm$ focal length

in air the focal length is $-\frac{1}{2}(\frac{1}{R_1}+\frac{1}{R_2})=-40$

$\frac{1}{R_1}+\frac{1}{R_2}=80$

when kept in liquid

$\frac{1}{f}=\frac{1-2}{-R_1}+\frac{2-1}{R_2}=(\frac{1}{R_1}+\frac{1}{R_2})=80$

as $f>0$ it behaves as convex lens , hence option $A$ is correct