A 55 year old man can focus objects clearly from 100 cm to 300 cm. Representing the eye as a simple lens 2 cm from the retina. What strength lens (focal length) must he wear in the lower part of his bifocal eyeglasses to focus at 25 cm?
Text Solution
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In order to see an object at 25 cm clearly, the dioptric power should be
Φ = 
+ 
= 
+ 
= 54 diopters,
and the combined power is the sum of the powers of the eye and the glasses,
Φ = Φ eye = Φ glasses .
Thus Φ glasses = Φ – Φ eye = 54 – 
= 3 diopters ( Φ eye = 
). He must wear 300º far-sighted eyeglasses. The corresponding focal length is
F glasses = 
= 
m = 33.3 cm.
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