A farsighted eye is corrected by placing a converging lens in front of the eye.

Question

A farsighted eye is corrected by placing a converging lens in front of the eye. This results in a virtual image located at the eye's near point, the distance to the closest object that can be seen clearly in focus by the eye. Normally, this is taken to be about 25.0 cm for someone with perfect eyesight. However, if a person has a near point of 35.5 cm, what focal length should contact lenses have to treat this farsightedness? For contact lenses, the distance from the eye to the contact lens is negligible. Focal length of lenses What is the corresponding power of the contact lenses? Power of lenses

Explanation / Answer

Given that

distance near point D=35.5 cm

distance d=25 cm

basing on the concept of defects of vision

now we find the focal length

1/f=1/0.25-1/0.355

1/f=0.105/0.089

focal length f=0.848 m=84.8 cm

now we find the power of the lens

power P=1/f=1/84.8=0.012 diopters

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