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Vision- is the passage of rays of light from an object through the cornea, acqueous humor, lens and vitreous humor to the retina and its appreciation in the cerebral cortex.
Normal (emmetropia) when rays coming from an object at a distance of 6 meters or 20 feet or more are brought to a focus in front on the retina by the lens
Abnormal- ametropia
1.Nearsightedness( myopia)- rays of light coming from an object at a distance of 6 meter (20) feet or more are brought to a focus in front of the retina. The correction is concave.
2. Farsightedness(hyperopia)-rays of the light coming from an object at a distance of 6 meter (20 ) feet or more are brought to a focus in back of the retina. Correction is convex lens
Accommodation-focusing the apparatus of the eye adjust to objects at different distances by means of increasing the convexicity of the lens
Presbyopia-elasticity of the lens decreases with increasing age. requires lenses to correct the problem