Distinguish between Farsightedness and Nearsightedness.



There are different types of refraction defects. These include Farsightedness and Nearsightedness. In the case of nearsightedness, also known as myopia, the eye can see nearby objects clearly but distant objects appear indistinct. This is because the far point of the eye is closer to the eye instead of at the infinity. During this eye defect, the image of a distant object form in front of the retina. The defect is caused by two main reasons. They are: 1) The curvature of the cornea and the eye lens increases.The muscles near the lens can not relax so that the converging power of the lens remains large.

2. The eyeball elongates so that the distance between the lens and the retina increases.

This eye defect can be rectified with the help of spectacles with concave lenses, which have proper focal length. These lenses diverge the incident rays and these diverged rays are then converged by the lens in the eye to form the image on the retina. Because the focal length of a concave lens is negative, a lens with negative power is required to correct nearsightedness. The power of the lens is different for different eyes depending on the magnitude of their nearsightedness.

Meanwhile, for farsightedness, you would be able to see distant objects clearly, but cannot see nearby objects that distinctly. This is because the near point of the eye is no longer at 25 cm but shifts farther away. During this defect the images of nearby objects get formed behind the retina.

The reasons for farsightedness are two:

1. The converging power of the lens becomes less as the curvature of the cornea and the eye lens decreases.

2. Due to the flattening of the eyeball the distance between the lens and retina decreases.

Now to correct this defect a convex lens with proper focal length has to be used. This lens converges the incident rays before they reach the lens and the lens, then converges them to form the image on the retina. The focal length of a convex lens is positive and for this reason the spectacles used to correct farsightedness has positive power. The power of these lenses is different depending on the extent of farsightedness.

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