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.