Question

A person cannot see distinctly objects kept beyond 2m.This defect can be corrected by using a lens of what power?

Answer 1

this defect is myopia.
It can be defined is as follows.
The eye defect in which a person cannot see objects beyond near point or far objects but can see the near objects clearly is called Myopia.
The point of maximum distance at which the eye lens can form an image on the retina is called ' far point '
This defect can be corrected by using biconvex lens.

Given distance of far point (D) = 2m
Let f be the focal length of eye lens.
Image distance ( distance between eye lens and retina ) for our human eye is always constant and is about 2.5 cm.
This is due to the action of ciliary muscles.
applying lens formula, we get
f = -D
= -2 cm.
but , the power of a lens is reciprocal of focal length.
hence. Power = 1/f. ( f in meters )
= 1/-2
= - 0.5 dioptre.


hope this helps you.
REGARDS

Answer 2

This defect is called MYOPIA(Near-sightedness).A person with myopic eye can see near by objects clearly .But cannot see distant objects distinctly. This defect can be corrected by using a concave lens of suitable power .(P=1/f)I.e 1/-2=0.5D (Diopter)
extra,
This defect mayarises due to:-
(i)Excessive curvature of eye lens or
(ii) elongation of eyeball.

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