12/14/2023 0 Comments Eye rods and cones diagramCones respond in intense light and are responsible for color vision. Rods respond in low light and can detect only shades of gray. Rods and cones are photoreceptors in the retina. In this case, images in the distance are blurry but images nearby are clear. Myopia (nearsightedness) occurs when an eyeball is elongated and the image focus falls in front of the retina. For both defects, images in the distance are clear but images nearby are blurry. Presbyopia is a deficit similar to a different type of farsightedness called hyperopia caused by an eyeball that is too short. Presbyopia occurs because the image focuses behind the retina. With age comes the loss of the flexibility of the lens, and a form of farsightedness called presbyopia results. The lens is operated by muscles that stretch it flat or allow it to thicken, changing the focal length of light coming through it to focus it sharply on the retina. The lens is dynamic, focusing and re-focusing light as the eye rests on near and far objects in the visual field. The main function of the lens is to focus light on the retina and fovea centralis. In conditions of low light, the iris relaxes and the pupil enlarges. In conditions of high ambient light, the iris contracts, reducing the size of the pupil at its center. The iris, which is conspicuous as the colored part of the eye, is a circular muscular ring lying between the lens and cornea that regulates the amount of light entering the eye. The cornea, the front transparent layer of the eye, and the crystalline lens, a transparent convex structure behind the cornea, both refract (bend) light to focus the image on the retina. It passes through other layers that process it so that it can be interpreted by the retina (Figure 1b). But light does not impinge on the retina unaltered. The photoreceptive cells of the eye, where transduction of light to nervous impulses occurs, are located in the retina (shown in Figure 1) on the inner surface of the back of the eye. The human eye is an organ that reacts with light and allows light perception, color vision, and depth perception.
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