Question

The near point of a hypermetropic person is 50 cm. The focal length of a convex lens used in his spectacles should be :

• A. 25 cm
• B. 30 cm
• C. 40 cm
• D. 50 cm

Answer 1

The correct option is D

50 cm

Correct answer explanation:

The normal eye has a near point as, $u=-25cm$.

The near point of the hypermetropic person is $v=-50cm$

So, the focal point of the lens in his spectacles should be,

$$\begin{gathered} \frac{1}{f}=\frac{1}{v}-\frac{1}{u} \\ \frac{1}{f}=\frac{1}{-50}-\frac{1}{-25} \\ \frac{1}{f}=\frac{-1+2}{50} \\ f=50cm \end{gathered}$$

Thus, the correct option is (d).