Question

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

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

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}=100\,\,cm $