Question

The radius of the convex surface of planoconvex lens is $20\,cm$ and the refractive index of the material of the lens is $1.5$. The focal length is:

• A. 30 cm
• B. 50 cm
• C. 20 cm
• D. 40 cm

Answer 1

Answer: (D)

40 cm

Answer 2

Using the relation for focal length of Plano convex lens
$\frac{1}{f}=(\mu-1)\left(\frac{1}{R_{1}}-\frac{1}{R_{2}}\right)$
(Refractive index of material of lens $\mu=1.5,\left.R_{1}=20\, cm , R_{2}=\infty\right)$
$\frac{1}{f}=(1.5-1)\left(\frac{1}{20}-\frac{1}{\infty}\right)$
$\frac{1}{f}=\frac{1}{40}$
or $f=40\, cm$