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4. An elderly person's range of vision is between 70 cm and 300 cm from the...

4. An elderly person's range of vision is between 70 cm and 300 cm from the eye. What focal length lens is needed to allow her to read a newspaper clearly at the normal distance of 25 cm from her eyes?

A. 38.9 cm B. 18.4 cm C. 27.3 cm D. 23.1 cm E. 70.0 cm

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Answer #1

Solution : near point of the person is shifted from 25 cm to 70 cm.

So object distance = -70 cm

Image distance = -25 cm

Now using lens formula

f = UV/ u- v

So f = - 25×-70 /(-25 - - 70) = 38.88 cm = 38.9 cm approx

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