Question

What is the refractive index of material of a planoconvex 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 $\frac{4}{3}$

According to lens maker's formula,
$\frac{1}{f}=(\mu -1)(\frac{1}{R_1}-\frac{1}{R_2})$
Here, f = 30 cm, $R_1=\infty$ (for plane surface.)
$R_2=-10cm$
$\frac{1}{30}=(\mu -1)(\frac{1}{\infty }-\frac{1}{-10})$
$\frac{1}{30}=\frac{(\mu -1)}{10}\mu =\frac{4}{3}$