Question

A plane convex lens is made of refractive index $1.6$ . The radius of curvature of the curved surface is $60cm$ . Then, the focal length of the lens will be

• A. $400cm$
• B. $200cm$
• C. $100cm$
• D. $50cm$

Answer 1

Answer: (C)

Refractive index $\mu =1.6$ Radius of curved surface $R_1=60$ Radius of plane surface $R_2=\infty$ The relation for focal length $n$ $\frac{1}{f}=(\mu -1)\left(\frac{1}{R_1}-\frac{1}{R_2}\right)$ $=(1.6-1)\left(\frac{1}{60}-\frac{1}{\infty }\right)$ $=0.6\times \frac{1}{60}=0.01$ or $f=\frac{1}{0.01}=100cm$