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Q.
A lens behaves as a converging lens in air and diverging lens in water. The refractive index of the material of the lens is
AFMCAFMC 2009Ray Optics and Optical Instruments
Solution:
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.