Question

The focal length of a thin lens made from the material of refractive index $1.5$ is $15 \,cm$. When it is placed in a liquid of refractive index $4 / 3$, its focal length will be _______$cm$.

• A. 80.31
• B. 50
• C. 78.23
• D. 60

Answer 1

Answer: (D)

$\frac{1}{15}=(1.5-1)\left(\frac{1}{R_{1}}-\frac{1}{R_{2}}\right)$
$\frac{1}{f}=\left(\frac{1.5}{4 / 3}-1\right)\left(\frac{1}{R_{1}}-\frac{1}{R_{2}}\right)$
$\frac{f}{15}=\frac{1 / 2}{(1 / 8)}$
$f =4 \times 15=60 \,cm$