Question

Far point of a person is 5 metres. In order that he has normal vision what kind of spectacles should he use

Answer 1

x = distance of far point = 5 meters

f = focal length of the correcting lens

for the lens :

v = image distance = - x

u = object distance = ∞

using the lens equation

1/f = 1/v + 1/u

1/f = 1/(- x) + 1/∞

taking inverse both side

f = - x

f = - 5 m

since the focal length is negative , hence the type of lens is concave lens. since focal length for concave lens is negative.

Answer 2

Hey mate,

◆ Answer-
Concave lens of focal length -5 m.

◆ Explanation-
# Given-
far point = 5 m

# Solution-
- Decrease in far point of vision in person suggests the person has myopia.
- Myopia is corrected with spectacles having concave lens of appropriate focal length.
- As far point is 5 m here, we'll use spectacles with concave lens of focal length -5 m in given patient.

Hope it helps you...