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State the working principal of the human eye.
The Human Eye is a natural optical instrument that enables human beings to see. The human eye works on the principle of refraction of light. It is like a camera that has a lens and screen system. Like a camera, the human eye takes in the light and forms the image on the screen. This image is detected by cells inside the eye and they transmit the information to the brain.
The following are the main parts of the human eye
Cornea: It is a thin membrane through which light enters. It forms the transparent bulge on the front of the eyeball. Most of the refraction occurs at the outer surface of the cornea.
Pupil: Pupil is the round black spot in front of the eye. It regulates the amount of light entering the eyes. Pupil works like the aperture of a camera. In the case of dim light, pupil dilates to allow more light to enter the eyes. In case of strong light, pupil constricts allowing less light to enter.
Iris: Iris is made of muscles that control the size of the opening of the pupil.
Lens: Lens lies just behind the pupil. The lens becomes thin to increase its focal length. This enables us to see distant objects clearly. To focus on nearer objects, the lens becomes thick to decrease its focal length. But there is a limit. The minimum distance of clear vision is 25 cm. Below this distance, we cannot see things clearly.
Retina: Retina works like a screen or camera film. The retina is full of light and color sensitive cells. These cells, upon receiving images send electrical signals to the brain, which processes this information to make a mental image of what we see.
The photoreceptor cells in the eye are of two types, viz. rod cells and cone cells. The rod cells are sensitive to dim light. The cone cells are sensitive to bright light and color.
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