Vision Defects of Human Eye
The image of an object on the retina is inverted real image, but the brain is trained to perceive the object correctly.
The light sensitive cells of retina get activated upon illumination and generate electrical signals. These signals are sent to the brain via the optic nerves, which in turn are interpreted as the information to perceive the objects.
Vision Defects: Elongated or flattened eyeballs of human eye leads to defects of vision
called myopia or short sightedness, hypermetropia or long–sightedness and
Presbyopia.
Myopia arises due to slight elongation of the eyeball. Because of the greater distance between the eye lens
and the retina, the image of a distant object is not formed on the retina but slightly in front of the retina. Usage of concave
lens of appropriate focal length can reduce the defects of Myopia.
An elongated eyeball leads to hypermetropia. Since the distance between the eye lens and the retina is less, the image of a distant object is formed slightly behind the retina when the ciliary muscles are relaxed. The muscles can reduce the focal length of the eye lens by compressing it so that the image is formed on the retina to view distant objects. However, for objects closer to the eye, the ciliary muscles cannot make the focal length of the eye lens sufficiently small. Hence image cannot be seen clearly. This defect is corrected by using a convex lens in front of the eye.
As a person grows old, the ciliary muscles become weak. The power of accommodation, thus, reduces. Distant objects can still be seen clearly. However, the muscles cannot accommodate sufficiently when one tries to focus on objects closer to the eye. This defect is known as presbyopia .