Question

Raj cannot see any object clearly beyond 40 cm distance from his eye.Find the focal length and power of the lens he needs in his spectacles for normal vision.

Answer 1

Final Answer :

Focal Length : 40 cm,

Power of Lens : 2.5 D

Steps:

1) According to the question,

Far -Point of Raj is 40 cm.

So, He can't see objects clearly after 40 CM.

So, We need to provide him lens which can bring object at infinity to its far -point.

2)

$u = \infty \\ v = 40 \: cm \\ \frac{1}{v} - \frac{1}{u} = \frac{1}{f} \\ = > \frac{1}{40} - \frac{1}{ \infty } = \frac{1}{f} \\ = > f = 40 \: cm$

So, Power of Lens :

P = 1/f(in m) = 1/ 0.4 = 2.5 D

So, Lens used is convex. (Focal length is positive)

Answer 2

hope it helps.................