Question

a long sighted person has a maximum distance of distinct vision of 50 cm. what kind of lens must be used in order to reduce the distance to 25 cm? what is the focal length of this lens?

Answer 1

u = -50 cm

v  = -25 cm

and we know that

1/f = 1/v - 1/u

or

1/f = 1/-50 - 1/-25 = 1/25 - 1/50

or

1/f = (2 - 1) / 50 = 1/50

thus, the focal length of the lens should be

f = +50 cm

so, the lens shall be convex lens

Answer 2

Explanation:

A long sighted person means that he can see distant object clearly but he cannot see objects near to him . This defect is also known as Hypermetropia. focal length=? Hence convex lens of focal length 50cm should be advised or used to correct this defect.