Passage For a person with perfect vision, light from an object is properly refracted by the eye lens to converge on a single point at the retina, forming a clear image of the object. Vision defects result from eye shape abnormalities or errors in the refractive power of the eye lens. Myopia (nearsightedness) occurs when light from a distant object is incorrectly focused in front of the retina. Hyperopia (farsightedness) occurs when light rays from a nearby object are focused beyond the retina.Many optical techniques are available to measure the refractive error of an individual to determine the necessary correction. Photorefraction is a photographic technique often used with young children because it does not require the individual to be still for a lengthy duration. When the patient is looking at the camera, a flash photograph is taken of the eye to determine the amount of light that is reflected off the retina and captured by the camera lens.In healthy eyes, all the light from the flash that enters the eye is reflected off the retina and returns back to the camera's light source. Because the camera lens does not receive this light, the pupil is completely dark in the resulting image. A myopic eye cannot properly focus the light at the retina. Due to the geometry of the eye and its lens, some of the light is reflected to the top portion of the camera lens. The camera captures an image of a pupil with a crescent of light at the top. In a hyperopic eye, the crescent appears at the bottom of the pupil. Ray diagrams for photorefraction are shown in Figure 1. Figure 1 Paths of light in photorefraction for different eyes: (A) Healthy, (B) Myopic, and (C) Hyperopic. HC. Howland, "Optics of photorefraction: orthogonal and isotropic methods." ©1983 Optical Society of America. -Myopia can be corrected with a:

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11ecba2d_12ce_a575_a3bf_2d54a4f16da7_MD0008_00 A clear image is formed in the eye when light from an object is refracted by the eye lens to focus at the retina. Refraction is the bending of light rays when light passes from one medium to another. Eyesight defects can be corrected with thin lenses. A converging (convex) lens converges parallel rays of light toward its focal point. A real image is formed from the convergence of the refracted light rays on the side of the lens opposite the object. Real images are always inverted (upside-down with respect to the object). A diverging (concave) lens spreads parallel rays of light away from its focal point. A virtual image is formed from the apparent convergence of refracted light rays traced back to the same side of the lens as the object. The image is virtual because the light rays do not actually converge at the image (Choice A). Virtual images are always upright. Myopia occurs when light rays from distant objects are focused in front of the retina. This condition is corrected with diverging lenses, which spread light before they reach the eye to move the location of the focused image farther back in the eye onto the retina. Diverging lenses create upright virtual images. (Choices C and D) Hyperopia causes light rays from nearby objects to be focused behind the retina. Converging lenses correct hyperopia by bending light rays inward before they reach the eye. Educational objective: Myopia is a condition in which light rays from distant objects are focused in front of the retina. Myopia can be corrected using diverging lenses, which spread light rays before they reach the eye. A diverging lens creates upright virtual images.

Passage For a Person with Perfect Vision, Light from an Object