Question

A lens behaves as a converging lens in air and diverging lens in water. The refractive index of the material of the lens is

• A. equal to that of water
• B. less than that of water
• C. greater than that of water
• D. Nothing can be predicted

Answer 1

Answer: (B)

If a lens (made of glass) of refractive index ${\mu }_g$ is immersed in a liquid of refractive index ${\mu }_l$, then its focal length in liquid $f_l$ is given by
$\frac{1}{f_l}=\left({}_l{\mu }_g-1\right)\left[\frac{1}{R_1}-\frac{1}{R_2}\right]...(i)$
If ${\mu }_g<{\mu }_l$, then $f_l$ and $f_a$ have opposite signs and the nature of lens changes ie, convex lens diverges the light rays and concave lens converges the light rays.