Question

A person can see clearly only upto a distance of 25 cm. He wants to read a book placed at a distance of 50 cm. What kind of lens does he require for his spectacles and what must be its power

• A. Concave – 1.0 D
• B. Convex + 1.5 D
• C. Concave– 2.0 D
• D. Convex + 2.0 D

Answer 1

Answer: (C)

Concave– 2.0 D

Answer 2

For correcting myopia, concave lens is used and for lens.

u = wants to see $= - 50cm$

v = can see $= - 25cm$

From $\frac{1}{f} = \frac{1}{v} - \frac{1}{u} \Rightarrow \frac{1}{f} = \frac{1}{- 25} - \frac{1}{( - 50)} \Rightarrow f = - 50cm$

So power $P = \frac{100}{f} = \frac{100}{- 50} = - 2D$