Calculate the focal length of a reading glass | vedclass

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Question

(B) The person has a near point of $ 75 \ cm $. To read at a normal distance of $ 25 \ cm $,the lens must form a virtual image of an object placed at

Vedclass · Accepted Answer

$37.5$

Vedclass · Answer

(B) The person has a near point of $ 75 \ cm $. To read at a normal distance of $ 25 \ cm $,the lens must form a virtual image of an object placed at $ 25 \ cm $ at the person's near point of $ 75 \ cm $.Given: Object distance $ u = -25 \ cm $,Image distance $ v = -75 \ cm $.Using the lens formula: $ \frac{1}{f} = \frac{1}{v} - \frac{1}{u} $$ \frac{1}{f} = \frac{1}{-75} - \frac{1}{-25} $$ \frac{1}{f} = -\frac{1}{75} + \frac{3}{75} = \frac{2}{75} $$ f = \frac{75}{2} = 37.5 \ cm $Thus,the focal length of the reading glass is $ 37.5 \ cm $.

Calculate the focal length of a reading glass of a person if his distance of distinct vision is $ 75 \ cm $. (in $cm$)

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