Question

A biconvex lens has a focal length $2/3$ times the radius of curvature of either surface. The refractive index of the lens material is

• A. $1.75$
• B. $1.33$
• C. $1.5$
• D. $1.0$

Answer 1

Answer: (A)

$1.75$

Answer 2

Using lens makers formula, $\frac{1}{f} =\left(\mu-1\right)\left(\frac{1}{R_{1}}-\frac{1}{R_{2}}\right)$. Here, $f =\frac{2}{3}R$, $R_{1} = +R, R_{2}=-R$ $\therefore\frac{1}{\left(23\right)R} = \left(\mu-1\right)\left(\frac{1}{R}+\frac{1}{R}\right)$ $=\frac{ \left(\mu-1\right)\times2}{R}$ $\Rightarrow \mu = 1.75$