Defects Of Vision human eye is also the related topic to the and optics
The inability of the eye to see the image of objects clearly is
called defect of vision. The defects of vision arise when the
eye lens is unable to accommodate effectively. The images
formed are therefore blurred.
Some people cannot see distant objects clearly without the
aid of spectacles. This defect of vision is known as short sight
or near-sightedness and it 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 glass 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.
The disability of the eye to form distinct images of nearby
objects on its retina is known as farsightedness. 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 a blurred image behind the retina.
This defect can be corrected with the aid of a suitable
converging lens.The lens refracts the light rays
and 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.
For your information
A thin film can be placed on the lenses of eyeglasses to keep them from reflecting wavelengths of light that are highly visible to the human eye. This prevents the glare of reflected light.