Vision Defect Presbyopia

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2.2.2 Defect visions When light rays do not converge on the retina precisely, several different types of vision defects may occur. For example, if the eyeball is too short from front to back, the rays converge at a point beyond the retina. This defect is called hypermetropia, or far-sightedness. Eyeglasses with convex lenses, which increase the rays’ convergence and thus pull the focus back onto the retina, can correct it. If the eyeball is too long from front to back, then the rays converge in front of the retina. This condition is called myopia, or near-sightedness. Eyeglasses with concave lenses can correct it. Another vision defect, presbyopia, is due to hardening of the lens, associated with aging. As the lens hardens, it becomes…show more content…
He noted that there are color combinations that we never see, such as reddish-green or yellowish-blue. Opponent-process theory suggests that color perception is controlled by the activity of three opponent systems. In the theory, he postulated about three independent receptor types which all have opposing pairs: white and black, blue and yellow, and red and green. These three pairs produce combinations of colors for us through the opponent process. Furthermore, according to this theory, for each of these three pairs, three types of chemicals in the retina occur, in which two types of chemical reactions exist. The colors in each pair oppose each other. Red-green receptors cannot send messages about both colors at the same time. This theory also explains negative afterimages; once a stimulus of a certain color is presented, the opponent color is perceived after the stimulus is removed. For example, red creates a positive response while green creates a negative response. These responses are controlled by opponent neurons, which are neurons that have an excitatory response to some wavelengths and an inhibitory response to wavelengths in the opponent part of the spectrum. According to this theory, color blindness is due to the lack of a particular chemical in the
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