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Question: (a) A person cannot read a newspaper placed nearer than \(50cm\) from his eyes. Name the defect of v...

(a) A person cannot read a newspaper placed nearer than 50cm50cm from his eyes. Name the defect of vision that he is suffering from. Draw a ray diagram to illustrate this defect. List its two possible causes. Draw a ray diagram to show how this defect may be corrected using a lens of appropriate focal length.
(b) We see advertisements for eye donation on television or in newspapers. Write the importance of such advertisements.

Explanation

Solution

Eye defects usually occur when the eye lens loses its power of accommodation and cannot focus the light rays coming from an object properly on the retina. These defects can be corrected by using corrective lenses which deviate the rays so that the eye lens can focus the deviated rays onto the retina.

Complete step by step answer:
If a person cannot read a newspaper placed nearer than 50cm50cm, this means that the near point of his eye has increased from normal (25cm)\left( \approx 25cm \right) and he cannot focus the light rays from an object near than that onto the retina properly.
This defect of vision is known as hypermetropia or far-sightedness. In this defect, the near point of the eye (the least distance from the eye beyond which the eye can see objects clearly) increases. Rays coming from an object closer than the near point are focused beyond the retina and a clear image is not formed as evident from the ray diagram below.

Two possible causes of hypermetropia are:

  1. The eyeball becomes flatter, that is the curvature of the eyeball decreases and therefore, the retina eventually comes forward and the light rays from a nearby object are focused beyond it.
  2. This can also happen if the position of the eye lens changes and the eye lens becomes dislocated. This can happen due to an accident.
    Hypermetropia can be corrected by using a corrective convex lens of appropriate focal length. The convex lens converges the light rays coming from the object and assists the eye lens to focus the rays on the retina. In hypermetropia, the eye lens cannot converge the rays onto the retina alone and the corrective lens does part of the converging job and therefore, makes up for the loss in the power of convergence of the eye lens.
    A ray diagram will help to understand the correction better.

    (b) We see advertisements for eye donation on television and newspapers. These are very important as they make people aware of the concept of eye donation. Eye donation can help people who are blind to see the world when others donate their eyes to these people. More and more people are becoming aware of the concept of eye donation and participating in it actively.

Note: There is also another kind of defect of the eye known as myopia or short-sightedness in which the eye cannot focus the light rays of objects at very far distances correctly upon the retina. This decreases the far point of the eye. This happens due to the eyeball becoming curvier and the retina thus, moving back. The light rays are focused in front of the retina. Myopia can be corrected by a diverging lens, that is, a corrective concave lens.