Question

What is the refractive index of material of a plano-convex lens, if the radius of curvature of the convex surface is 10 cm and focal length of the lens is 30 cm?

• A. $\frac{6}{5}$
• B. $\frac{7}{4}$
• C. $\frac{2}{3}$
• D. $\frac{4}{3}$

Answer 1

The correct option is D $\frac{4}{3}$

Given: The focal length of the plano-convex lens is $30cm$.

The radius of curvature of the convex surface is $10cm$

To find: The refractive index of the lens material.

The relation between the focal length and the radius of curvature for the lens is given by:

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

For plano-convex lens the radius of curvature for plane-surface is infinite.

So, $R_1=+10,R_2=\infty$

and $f=30$

Substitute the values.

$\frac{1}{30}=(n-1)(\frac{1}{10}+0)$

$3n-3=1$

$n=\frac{4}{3}$