Question

If a convex lens of refractive index $1.44$ is dipped in liquid of refractive index $1.49$, then it behaves as

• A. concave lens
• B. convex lens
• C. mirror
• D. None of these

Answer 1

Answer: (A)

By lens formula
$ \frac{1}{f} = (n-1) \left(\frac{1}{R_1}-\frac{1}{R_2}\right)$
When lens is dipped in liquid, its refractive index
${}_{l} n_{g} = \frac{{}_{a} n_{g}}{{}_{a} n_{l}}$
Given, $ {}_{a} n_{g} =1.44,\, {}_{a} n_{l} = 1.49$
$\therefore {}_{l} n_{g} = \frac{1.44}{1.49} < 1 $
Hence, focal length $(f_1)$ of lens becomes negative.
$ \frac{1}{f_1} = ({}_{l} n_{g}-1) \left(\frac{1}{R_1}-\frac{1}{R_2}\right)$
Hence, lens behaves like a concave lens.