Ans. NEARSIGHTEDNESS
The disability of the eye to form distinct images of distant objects on its retina is known as nearsightedness.
This is also known as shortsightedness.
Near sightedness may be due to the eyeball being too long. Light rays from a distant object are focused in front of the retina and a blurred image is produced
The nearsighted eye can be corrected with gla"ss or contact lenses that use diverging lenses. Light rays from the distant objects are now diverged by this lens before entering the eye. To the observer, these light rays appear to come from far point and are therefore focused on the retina, thus forming a sharp image.
FARSIGHTEDNESS
The disability of the eye to form distinct images of nearly objects on it retina is known as farsightedness.
it is also known as longsightedness.
When a farsighted eye tries to focus on a book held closer than the near point, it shortens its focal length as much as it can. However, even at its shortest, the focal length is longer than it should be. Therefore, the light rays from the book would form sharp image behind the retina. f
This defect can be corrected with the aid of a suitable converging lens (Fig.12.36-b). The lens refracts the light rays arid they converge to form an image on the retina. To an observer, these rays appear to come from near point to form a sharp virtual image on the retina.