Question

Noopur needs a lens of power 4.5D in order to correct her vision. What kind of defect in vision is she suffering from?
A. Myopia
B. Hypermetropia
C. Astigmatism
D. None of these

Answer 1

Hint: From the given value of power of corrective lens, you could find whether the focal length of the corrective lens is positive or negative. Thereby, we would be able to identify the lens used so as to correct the defect. Now, you may recall which defect is corrected using that particular lens and hence find the answer.

Complete answer:
In the question we are given that for Noopur to correct her vision, the power of a lens should be 4.5D. From this given information we have to find what kind of defect she is suffering from.
Firstly, let us recall what the power of a lens is.
The power of a lens (P) is defined as the reciprocal of the focal length (f) of that lens (in meters), that is,
$P=\dfrac{1}{f}$
So, if the power of a lens is positive then that would mean the focal length of the lens is also positive.
But we know that the focal length is positive for a convex lens and hence we get that the corrective lens used by Noopur is convex. Also, we know that convex lenses are used by people who suffer from long sightedness or hypermetropia. This is that eye condition in which the person has difficulty in seeing the nearby objects but can very clearly see the objects that are far away.
Therefore, we found that Noopur here is suffering from hypermetropia.

Hence, option B is the correct option.

Note:
Hypermetropia is caused when the eye focuses the light on the back of the eye behind the retina. Light is focused on the retina for a person with normal vision. This may be as the result of your eyeballs being shorter or due to the cornea being flat or even due to genetic reasons. By using the corrective lens the patient will be able to focus light back on the retina.