As soon as you step into a dark room from a lighted one you will realise that you are not able to see anything. It takes time for your eyes to get adjusted to the darkness. At first everything appears blurred and then gradually you begin to see clearly. Do you know why this happens? Actually, our eyes are capable of seeing in bright light as well as in dim light. The pupil of our eye adjusts its size according to the brightness of light. When we are in dim light, the pupil expands to let more light enter the eye. When we are in bright light, the pupil contracts. But when we move into a dark room from bright sunlight, the pupil takes some time to dilate. Therefore, it takes some time for things to become visible clearly. There is another important reason for this. The innermost layer inside our eye is called retina on which images of objects are formed. But we do not see the object until light-sensitive nerve endings on the retina send the brain a message along with the optic nerve. The coloured part of the eyes is called iris. This can open and close to let either more or less light pass through the pupil. The retina is composed of many cells called rods and cones. These rods contain a purple-coloured substance called rhodopsin. When light coming from an object falls on the retina, it splits rhodopsin into two substances. The splitting of rhodopsin produces electrical signals that go to the brain and we see the object. If rhodopsin is continuously decomposed by the light entering our eyes, a time could come when the whole stock of rhodopsin would be decomposed. And in this condition the person would go blind. But actually, this never happens because rhodopsin is simultaneously decomposed and formed. Therefore, the retina never suffers from a complete loss of rhodopsin. In bright light, our eyes may feel fatigued, but will never become blind. A lot of rhodopsin is decomposed in bright light. In this condition, if we suddenly enter a dark room we cannot see things clearly because the amount of rhodopsin is reduced. It takes some time for the rhodopsin to form again, after which things become clearly visible to us. In the opposite case, when we suddenly enter into bright light from a dark room, our eyes get dazzled. The reason is that in a dark room very less rhodopsin gets decomposed. But on a sudden exposure in bright light a lot of rhodopsin gets washed. And thus, the intensity of the electric impulses reaching the brain increases and our eyes get dazzled. Vitamin A is required for the production of rhodopsin. A deficiency of this vitamin can cause night blindness. Vitamin A is found in leafy vegetables, carrots, eggs and milk.
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