Test: Defects of Vision - UPSC

Hypermetropia can be corrected by using a convex lens. A convex lens converges the incoming light such that the image is formed on the retina.

In myopia, u = infinity , v = d = distance of far point
By 1/F = 1/v - 1/u
We get  f = -d
Since the f is negative, the lens used is concave.

The near point of the eye is the minimum distance of the object from the eye, which can be seen distinctly without strain. For a normal human eye, this distance is 25 cm. The far point of the eye is the maximum distance to which the eye can see the objects clearly. The far point of the normal human eye is infinity.

Given ,v = -3m or -300cm
u = -infinity
f = ?
according to lens formula ,
1/f = 1/v - 1/u
1/f = -1/300 - (-1/infinity)
1/f = -1/300 + 1/infinity
focal length = -300 cm or -3metre
now,
power = 1/focal length(metre)
power = -1/3
power = -0.33 dioptre

Myopia is an eye defect in which light coming from an object gets focused slightly in front of the retina instead of directly falling on the retina. In this defect, a person cannot see distant objects, but can see the objects closer to him clearly. It is also known as Nearsightedness or Short Sightedness.
Myopia is caused due to the variation in the distance between the eye lens and retina. This distance increases when the eyeball gets elongated along its axis. This results in the object at a distance to appear blurry.
Thus, myopia is due to elongation of the eyeball.

The focal length of the concave lens should be -1 m as object at infinity forms image at focus.

Surgery to treat cataracts involves removing the clouded lens and replacing it with a synthetic new one. This procedure is safe and very effective.They're researching eye drops that may dissolve cataracts so patients don't have to go to surgery.

Bifocal lenses are the lenses which are used for the people who suffer from both defects of vision that are hypermetropia and myopia, for those people who suffer from presbyopia or astigmatism. These lenses have two distinct powers and hence they serve two functions :- one is for distant vision and the other is for near vision. The lenses are divided into two parts. The upper part consists of concave lens which serves for distant vision at the lower part consists of convex lens which serves for near vision.

Short-sightedness (myopia) usually occurs when the eyes grow slightly too long, which means they're unable to produce a clear image of objects in the distance.

A defective eye cannot see close objects clearly because there image is formed beyond retina. This defect as known as farsightedness. Here rays from near object could not be converged at retina due to focal length of the eye lens could not be decreased. So, image is formed beyond retina.

Nearsighted individuals have image formed in front of the retina. They must correct the problem by wearing a lens which provides for some diverging of light prior to reaching the lens of the eye. This will move the image further from the lens of the eye and back towards the retina.

The most common are nearsightedness, farsightedness, astigmatism and presbyopia. Concave and convex lenses compensate for defects in the eye, bending light rays to place them precisely on the retin and correcting vison. A concave lens is thinner in the center and thicker on the edges. Concave lenses are used in the correction for near sightedness.

This person suffers from the defect of hypermetropia.
For him u = -30cm, v = -1.5 m = -150cm
Therefore, focal length of corrective lens to be used by him is
1/f = 1/v- 1/u = 1/-150 - 1/-30 = 4/150 = 37.5cm
The positive sign shows that the lens needed is a convex lens of focal length 37.5 cm.
Hence, power of lens needed
P =1/f = 100/37.5 = 2.67D

When object is placed at 25 cm, then a virtual image should form at 50 cm. 
So, u = -25 cm, v= -50 cm. 
So, 1/f = 1/-50 - 1/-25 
f = 50 cm = 0.5 m 
Power, P = 1/f = 1/0.5 =  +2D 
Therefore , a convex lens is needed.

She is suffering from hypermetropia in this case focal length of her eyeball is decreased and hence the image is formed behind the retina so she cannot see nearby objects as they r already close to her eyes and the image is formed behind the retina but she can see objects that r far as their image is formed on the retina

Option ( c) does not hold true for Shortsighteness because 
The image is formed in front of the retina.

As Myopia is a defect of vision in which a person cannot see the distant object clearly. This can be corrected by using the concave lens of appropriate focal length

Presbyopia occurs in old age when the eye lens hardens and loses its flexibility due to the weak ciliary muscle. In this situation the human eye becomes unable to focus on distant as well as near by objects. Hence option(d) is the correct answer.

For person affected with myopic condition, the image instead of forming on retina, it forms in front of it. In order to cure such type of defect, concave lens of power - 1.25D is used.

By far the most common and simplest treatment for presbyopia is bifocal or progressive lens eyeglasses. A bifocal lens is split into two sections. The larger, primary section corrects for distance vision, while the smaller, secondary section allows you to see up close.