Eye glasses for far sighted A 77 year old man has a near point of 110 cm. What p

Question

Eye glasses for far sighted

A 77 year old man has a near point of 110 cm. What power lens (in dioptres) does the man need in order to be able to see an object clearly at the normal near point of 25 cm? Assume that the person will get contact lenses, that is neglect the two that glasses would be away from the eye. This is an example of farsightedness. The correcting lens (converging in this case) has to move the (virtual) image of an object placed at the normal near point (d_0) to the actual near point of the person (d_1). Then you can calculate focal length of the lens. Remember that the image is virtual.

Explanation / Answer

Apply lens equation

1/f = 1/p + 1/q

= 1/0.25 + 1/-1.1m

1/f = 3.09 m

Power of the lens is

P = 1/f = 3.09 m

positive sign indicates converging lens

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