Question

The far point of a myopic person is 80 cm in front of the eye. What is the nature and power of the lens required to correct the problem?

Answer 1

The person is suffering from an eye defect called myopia. In this defect, the image is formed in front of the retina. Hence, a concave lens is used to correct this defect of vision.

Object distance, u = infinity = ∞

Image distance, v = – 80 cm

Focal length = f

According to the lens formula,

`1/"v" - 1/"u" = 1/"f"`

`1/(-80) - 1/(-∞) = 1/"f"`

`1/(-80) = 1/"f"`

f = −80 cm = −0.80 m   ...(concave lens)

∴ power of modified lens,

P = `1/"f"`

= `1/(-0.80  "m")`

= −1.25 D

A concave lens of power −1.25 D is required by the person to correct his defect.