The eye has a roughly spherical shape. The outer coat of the eye is white. It is tough so that it can protect the interior of the eye from accidents.
Its transparent front part is called cornea . Behind the cornea, we find a dark muscular structure called iris. In the iris, there is a small opening called the pupil. The size of the pupil is controlled by the iris. The iris is that part of eye which gives it its distinctive colour. When we say that a
person has green eyes, we refer actually to the colour of the iris. The iris controls the amount of light entering into the eye.
Behind the pupil of the eye is a lens which is thicker in the centre. The lens focuses light on the
back of the eye, on a layer called retina. The retina contains several nerve cells. Sensations felt by the nerve cells are then transmitted to the brain through the optic nerve. There are two kinds of cells–
(i) cones, which are sensitive to bright light and
(ii) rods, which are sensitive to dim light.
Cones sense colour. At the junction of the optic nerve and the retina, there are no sensory cells, so no vision is possible at that spot. This is called the blind spot.