We see objects around us with our eyes. The eye is an important sense organ. The human eye is roughly spherical in shape. Its transparent front part is called the cornea. Behind the cornea, there is a muscular structure called the iris. There is a small opening in the iris, whose size can be controlled, and is called the pupil. The iris controls the amount of light entering the eye. Behind the pupil, the eye has a lens, which is thicker in the middle. The lens converges light on the retina, which has a number of nerves. The nerves carry the sensations to the brain through the optic nerve. The nerves contain two types of cells - cones and rods. The cones are sensitive to bright light, while the rods are sensitive to dim light. The cones also distinguish the colour of an object. The small region where the optic nerve and the retina meet has no sensory cells, and is called the blind spot.
The impression of an image on the retina lasts for th of a second. If 16 still images of an object are flashed before the eye per second, then the eye perceives the object as moving. The eye lids protect the eye from dust. The eye has the ability to form a clear image of a near and a far object. This ability is called its power of accommodation. For a normal eye, the distance for clear vision is 25 cm. This differs from person to person, and varies with age. Some persons can see near objects clearly and some others far off objects. These defects can be corrected using suitable corrective lenses. The eye sight of some people becomes foggy in their old age, and objects appear dim, due to cataract. Some people have no vision since birth. Such people use Braille to read and communicate.