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:
far point of a normal eye = ∞
but this person can only seee upto 80cm from his eyes, not beyond that. So to be able to see objects beyond 80cm, let at ∞, the image needs to be formed at 80cm so that the person can see them distinctly
Therefore
Object distance, u=−∞
Image distance, v=−80 cm
Focal length = f
According to the lens formula,
1v−1u=1f
−180−1−∞=1f
f=−80 cm=−0.8 m
3. We know, Power,
P=1f(in meters)
P=1−0.8 m=−1.25 D
Negative sign shows that the lens required is concave.