Question

A person needs a lens of power -4.5 D for correction of her vision. What kind of defect in vision is she suffering from?
A. Myopia
B. Ptosis
C. Presbyopia
D. Diplopia

Answer 1

Hint:Use the formula for the power and find the focal length of the lens. Then from the focal length identify the type of lens. According to the type of lens, you can identify the eye defect from which the person is suffering.

Formula used:
$P=\dfrac{1}{f}$,
where $f$ is focal length and $P$ is power of the lens.

Complete step by step answer:
It is given that the person is suffering from an eye disease and he needs a lens whose power is -4.5 D to correct the defect in his vision.The power of a lens is defined to be the reciprocal of the focal length of the lens. Therefore, if we have a lens whose focal length is f, then the power of that lens is given to be equal to,
$P=\dfrac{1}{f}$ …. (i)
The unit of power of a lens is dioptre (D).
And $1D=1{{m}^{-1}}$.
It is given that the power of the required lens for the person is equal to -4.5 D.
This means that $P=-4.5D$
Substitute the value of P in equation (i).
$-4.5=\dfrac{1}{f}$
$\therefore f=\dfrac{1}{-4.5}=-0.22m$.
This means that the focal length of the lens must be -0.22m.Since the focal length of the lens is negative, the required lens is a diverging lens (concave lens). This means that the image is formed in front of the retina. Therefore, the person is suffering from myopia, where the image of the objects is formed in front of the retina instead of on the retina.

Hence, the correct option is A.

Note: Myopia is also called near-sightedness. In this defect, the person can see the near objects clearly but cannot see the far away objects clearly. Presbyopia is a defect in which the person can see the far away objects clearly but cannot see the near objects clearly. This is because the image is formed behind the retina and to correct this defect we need a converging lens (convex lens).