Question

A thin lens of refractive index 1.5 has a focal length of 15 cm in air. When the lens is placed in a medium of refractive index 4/3, its focal length will become

• A. 20 cm
• B. 30 cm
• C. 45 cm
• D. 60 cm

Answer 1

The correct option is D 60 cm

For air,
$\frac{1}{15}=(1.5-1)\left(\frac{2}{R}\right)...\left(1\right)$
For another medium,
$\frac{1}{x}=(\frac{1.5}{4/3}-1)\frac{2}{R}...\left(2\right)$
$\Rightarrow \frac{1}{x}=\frac{1}{8}\times \frac{2}{R}\Rightarrow \frac{1}{x}=\frac{1}{4R}$
From (1) and (2)
x = 60 cm