|Spanish - Chinese||The Optics Book - The human eye||Written by:Karen|
††††††††††† Accommodation is the process by which the lens changes shape to allow the eye to focus at different distances.† With age, accommodation becomes more difficult.† This is called presbyopia.† This is why many older people need reading glasses.
††††††††††† Nearsightedness may be caused by various things.† For instance, genetically, a personís eyeball may be to long, causing the rays to focus too far in front of the retina.† Nearsightedness, or myopia, is when the Light rays converge too far in front of the retina.† Corrective lenses for myopia are diverging lenses.† The diverging lens will cause the Light rays to bend outwards enough, so that when the rays hit the lens of your eye, they will be far enough apart to focus correctly onto the retina.
††††††††††† On the other hand, farsightedness can be caused by a personís eyeball being too short, which will cause the rays to focus in back of the retina.† Converging lenses are needed to correct hyperopia.† The converging lens will cause the rays to come in enough before they hit the lens, so that they will focus onto the retina properly.† Oftentimes, hyperopia is latent, or hidden, because the eye can accommodate enough to make up for the hyperopia.† Latent hyperopia is picked up when cycloplegic drops are used to stop accommodation.
††††††††††† Another error that the eye can have is astigmatism.† An irregularly shaped cornea or lens will cause astigmatism.† It makes objects appear to tilt.† Cylindrical lenses are needed to correct astigmatism.† The cylindrical lens causes certain Light rays to bend more than others.† This will counteract the irregularities of your cornea or lens.
|The Optics. Made by Karen, Timothy and, César for ThinkQuest . 1999 - 2000 All rights reserved|