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

Answer: (D)

$\frac{4}{3}$

Answer 2

According to lens makers formula
$\frac{1}{f} = (\mu -1)(\frac{1}{R_1} -\frac{1}{R_2})$
Here, $f= 30 \,cm, R_1 =\infty$ (For plane surface)
$R_2 = -10\,cm$
$\therefore \frac{1}{30}=(\mu-1)(\frac{1}{\infty} - \frac{1}{10})$;
$\frac{1}{30} = \frac{(\mu-1)}{10}$ or $\mu = \frac{4}{3}$