The various defects from which an eye can suffer
1. Hypermetropia or long sightedness.
2. Myopia or short sightedness.
1. Hypermetropia: A person suffering from this defect can see distantobjects clearly but cannot see nearby objects clearly. In this defect the near point lies farther away than 25 cm. When the relaxed eye produces an image of a distant object behind the retina the condition is known as hyperopia or hypermetropia, and the person is said to be farsighted or long sighted. With this defect, nearby objects are blurred. This defect is due to the following reasons:
Either the hyperopic eyeball is too short, or
(ii) The ciliary muscle is unable to change the shape of the lelis enough to properly focus the image i.e. the focal length of the less increases.
The condition can be corrected with a converging or convex lens. See Important Diagrams.
2. Myopia: A person suffering from myopia or short sightedness can see nearby objects clearly but cannot see the far away objects clearly.
This defect occurs either:
(i) When the eyeball is longer than normal or
(ii) When the maximum focal length of the lens is insufficient to produce a clearly formed image on the retina.
In this case, light from a distant object is focused in front of the retina. The distinguishing feature of this condition is that distant objects are not seen clearly.
Near sightedness can be corrected with a diverging- or concave lens.