Question

A person suffering from myopia (short-sightedness) can see clearly only up to a distance of 2m. What is the power of a concave lens required to rectify this defect?
(A). -0.5D
(B). -1.5D
(C). -2.0D
(D). -2.5D

Answer 1

Hint: Before attempting this question one must have a prior knowledge about the defect of myopia and also remember to take v as 2m, using these instructions will help you to approach towards the solution to the question.

 Complete step-by-step solution -
According to the given information Person is suffering from myopia which is a defect in which a person faces difficulty to see far objects This defect is corrected by using a concave lens which projects the image of far objects in the minimum distance a person can see.
Since it is given that person can see up to distance of 2m
Therefore position of image will be at 2m i.e. v = - 2m from eye of a person
So the position of object will be at infinity i.e. u = $\infty $
To find the focal length of concave lens let’s use the lens formula i.e. $\dfrac{1}{v} - \dfrac{1}{u} = \dfrac{1}{f}$
Substituting the given values in the above formula we get
$\dfrac{1}{{ - 2}} - \dfrac{1}{\infty } = \dfrac{1}{f}$
$ \Rightarrow $$\dfrac{1}{{ - 2}} = \dfrac{1}{f}$
$ \Rightarrow $f = – 2m
Now using power formula of lens i.e. \[P = \dfrac{1}{f}\]
Substituting the given values in the above formula we get
\[P = \dfrac{1}{{ - 2}}\] = – 0.5D
The power of the required lens is – 0.5D
Hence option A is the correct option.

Note: There are more eye defects like Myopia such hyperopia in which a person face difficulty in viewing the objects located near to him this defect can be corrected with wearing a contact lens of convex lens, cataract is also an eye defect in which a person face problem in viewing objects clearly and this defect can be corrected by the surgery this defect occurs in old age peoples. There are more defects of the eye like Astigmatism and presbyopia.