Question

A bi-convex lens of radius 60 cm and 30 cm is made of refractive index $\mu =1.5$, find the focal length of lens if surrounding medium is air.

• A. 0.6 m
• B. 1.5 m
• C. 0.9 m
• D. 1.2 m

Answer 1

The correct option is D 1.2 m

We know,
$\frac{1}{f}=(\mu -1)(\frac{1}{R_1}-\frac{1}{R_2})$

$\frac{1}{f}=(1.5-1)(\frac{1}{30}-\frac{1}{60})$
$\frac{1}{f}=0.5\times \frac{1}{60}=\frac{1}{120}$

$\Rightarrow f=120$ cm $=$ 1.2 m