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Paragraph 1:Have you ever noticed that when you walk into a dark movie theatre from the daylight, you are “blind” at first? It takes your eyes a while to adjust to the darkened room, and the same is true when you step out of the theatre into bright daylight. This phenomenon is called dark and light adaptation, and one cause of it is the nature of the cells in the human eye that make vision possible. These cells are called photoreceptors, because they receive light. They are divided into two types, rods and cones. The rods are extremely sensitive, and can be triggered by just a single photon of light. The cones, on the other hand, need brighter light, but they selectively receive this light in a way that allows us to see different colors. In dark conditions, our eyes are mainly using the rod photoreceptors, but when there is adequate light, the cones take over. This switch between rods and cones takes time, which is what causes the momentary blindness. Of course, the story is not as simple as that – other changes also need to take place. By contrast, the pupil of the eye opens wider in dark conditions, in order to let the maximum of light in, but contracts when there is too much light. All in all, vision is the product of an intricate and coordinated biological system that scientists are still working to understand.
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