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

Explanation

The person is suffering from a 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=`oo`
Image distance, v=-80cm
Focal length=f
According to the lens formula,
`1/v-1/u=1/f`
`-1/(80)-1/(oo)=1/f`
`1/f=-1/(80)`
f=-80cm=-0.8cm
we know
Power, P`=1/(f("in metres")`
`P=1/(-0.8)=-1.25D`
A concave lens of water -1.25 D is required by the person to correct his defect.