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? [3 MARKS]

Answer 1

Each point: 1 Mark

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.

2. Given:
Object distance, u = infinity

Image distance, v = - 80cm

Focal length = f

According to the lens formula,

$\frac{1}{v}$ - $\frac{1}{u}$ = $\frac{1}{f}$

-$\frac{1}{80}$ - $\frac{1}{\infty }$ = $\frac{1}{f}$

$\frac{1}{f}$ = -$\frac{1}{80}$

f = -80cm = -0.8m

3. We know, Power, P = $\frac{1}{f}$(in meters)

P = $\frac{1}{-0.8}$ = -1.25D

negative sign shows that the lens required is concave.