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| The Eye |
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| Our eye is the most important natural optical instrument. The eye is nearly spherical in shape with a slight bulge in the front part. |
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| Cornea |
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| The front part of the eye is covered by a transparent spherical membrane called the cornea. Light enters the eye through cornea. The space behind the cornea is filled with a liquid called aqueous
humor. |
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| Iris |
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| Just behind the cornea is a dark coloured muscular diaphragm which has a small circular opening in the middle. |
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| Pupil |
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| Pupil is the small circular opening of iris. The pupil appears black because no light is reflected from it. |
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| The iris regulates the amount of light entering the eye by adjusting the size of the pupil. |
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| Let us see how iris regulates the amount of light entering the eye. |
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 When the intensity of light is more or if it is a bright source of light then the iris makes the pupil to contract and as a result the amount of light entering the eye decreases. |
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 When the intensity of light is less or if the light is dim then the iris dilates the pupil so that more light can enter the eye. |
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| The eye lens is a convex lens made of a transparent jelly - like proteinaceous material. The eye lens is hard at the middle and gradually becomes soft towards the outer edges. The eye lens is held in position by ciliary muscles. The ciliary muscles help in changing the curvature and focal length of the eye - lens. |
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| The inner back surface of the eye ball is called retina. It is a semi-transparent membrane which is light sensitive and is equivalent to the screen of a camera. The light sensitive receptors of the retina are called rods and cones. When light falls on these receptors they send electrical signals to the brain through the optic nerve. The space between the retina and eye lens is filled with another fluid called vitreous
humor. |
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| It is a spot at which the optic nerve enters the eye and is insensitive to light and hence the name. |
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