Question

Person cannot see an object lying beyond 80 cm, where as a normal person can easily see the object distant 160 cm. the focal length and nature of the lens and to rectify this defect will be

Answer 1

Answer:

Given :-

u = - 160 cm

v = - 80 cm

Solution :-

Using, 1/f = 1/v - 1/u, we get

⇒ 1/f = 1/- 80 + 1/160

⇒ 1/f = - 2 + 1/160

⇒ 1/f = - 1/160

⇒ f = - 160 cm = 1.6 m

Hence, the focal length is - 160 cm or 1.6 m.

Power, P = 1/f(in metre)

⇒ P = 1/1.6

⇒ P = - 0.625 D

Hence, the power of the correcting lens is - 0.625 D.

Answer 2

Explanation:

As a person cannot see far objects , i.e beyond 80 cm, he is suffering from myopia.

Hence, by using lens formula, we can find the focal length and power of a correcting lens.

Given :-

u = - 160 cm

v = - 80 cm

Solution :-

Using, 1/f = 1/v - 1/u, we get

⇒ 1/f = 1/- 80 + 1/160

⇒ 1/f = - 2 + 1/160

⇒ 1/f = - 1/160

⇒ f = - 160 cm = 1.6 m

Hence, the focal length is - 160 cm or 1.6 m.

Power, P = 1/f(in metre)

⇒ P = 1/1.6

⇒ P = - 0.625 D

Hence, the power of the correcting lens is - 0.625 D.