Chapter 17 Errors of Refraction
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9 Terms
astigmatism
defective curvature of cornea or lens of eye
astigmatism results from
one or more abnormal curvatures of cornea or lens
causes light rays to be unevenly and not sharply focused on retina so image is distorted
astigmatism solution
cylindrical lens placed in proper position in front of eye in glasses
hyperopia (hypermetropia)
farsightedness
eyeball too short or refractive lens power too weak
hyperopia treatment
convex lens bends rays inward before they reach the cornea and rays can be focused properly on retina
myopia
nearsightedness
eyeball too long or lens refractive power too strong
myopia treatment
concave lens spread rays out before they reach cornea so they can be properly focused on retina
presbyopia
impairment of vision as a result of old age
lens cannot thicken to bend rays coming from near objects
presbyopia
convex lens refracts rays coming from objects closer than 20 ft
Note 
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