Question

The focal point of an equi- convex lens whose refractive index is $1.5$ is $10cm$ in air. Its focal point inside a liquid of refractive index $1.25$ is

• A. $20cm$
• B. $25cm$
• C. $30cm$
• D. $50cm$

Answer 1

The correct option is A $20cm$

Given that,

Focal length $f=10cm$

Refractive index ${\mu }_1=1.5$

Refractive index ${\mu }_2=1.25$

We know that,

$\frac{1}{f}=(n-1)(\frac{1}{R_1}-\frac{1}{R_2})$

$\frac{1}{10}=(1.5-1)(\frac{1}{R_1}-\frac{1}{R_2})....\left(I\right)$

$\frac{1}{f}=(1.25-1)(\frac{1}{R_1}-\frac{1}{R_2})....\left(II\right)$

Now, divided equation (I) by equation (II)

$\frac{f}{10}=\frac{0.5}{0.25}$

$f=20cm$

Hence, the focal length is $20cm$