2. Long-sightedness or Hypermetropia:
In this condition, the light is focussed behind the retina (the image is formed behind the retina). As a result, the distant objects are seen clearly, while the near objects appear blurred. Long-sightedness results due to shortening of the eyeball or the lens has become too thin.
This is a more complicated defect in vision. In this, the surface of cornea becomes irregular and therefore some of the light rays are focussed while others are not. As a result, some parts of the object appear blurred while other parts appear clear.
In this condition, the lens loses its flexibility and as a result the near objects cannot be seen clearly. This defect normally occurs in the older people.
5. Night Blindness:
In this condition, there is difficulty in seeing in the dim light (hence the name night blindness). This is because of non-production of rhodopsin pigment in the rod cells, which function in dim light. In the absence of rhodopsin these cells cannot function. Thus, there is lack of normal night vision. It is most often caused due to deficiency of vitamin A.
6. Colour Blindness:
In this condition, a person is unable to discriminate between red and green colours. This is a genetic defect about which you have already studied in the chapter on heredity and genetics (X-linked inheritance).
In this condition, the lens of the eye becomes opaque and as a result the vision is cut down.
Cataract can be treated by surgical removal of lens and using convex lenses in the spectacles which compensate for the removed lens. Nowadays, a small plastic lens is implanted behind the iris to correct the defect.
In this condition, either the two eyes somewhat converge (known as cross-eye) or diverge (known as wide-eye). In both these conditions, a person may have double vision. Squint in the eyes can be treated by surgery or by suitable exercises.