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February 14, 2014
Structure and Working of Eye - Simple Review
Light enters the eye through the cornea. After passing through the cornea, light travels through the pupil (the black dot in the middle of the eye). The iris—the circular, colored area of the eye that surrounds the pupil—controls the amount of light that enters the eye.
The pupil dilates (enlarges) and constricts (shrinks) like the aperture of a camera lens as the amount of light in the immediate surroundings changes. The iris allows more light into the eye when the environment is dark and allows less light into the eye when the environment is bright.
The size of the pupil is controlled by the action of the pupillary sphincter muscle and dilator muscle.
Behind the iris sits the lens. By changing its shape, the lens focuses light onto the retina.
The retina contains the cells that sense light (photoreceptors) and the blood vessels that nourish them. The most sensitive part of the retina is a small area called the macula, which has millions of tightly packed photoreceptors (the type called cones).
Light receptor cells called photoreceptors are two groups:
a) the rods and
b) the cones.
The rods respond to shades of light - they 'see' in black and white.
The cones respond to colours of light. There are 3 exciting varieties - blue, green and red. So the red cones respond to red light, and so on.
Each photoreceptor is linked to a nerve fiber. The nerve fibers from the photoreceptors are bundled together to form the optic nerve.
This retinal image is not the same as the object that is being looked at. The image is inverted.
The photoreceptors in the retina convert the image into electrical signals, which are carried to the brain by the optic nerve. The brain then translates the electrical signals into the images we see.