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A short-sighted person can read a book ...

A short-sighted person can read a book only up to 15 cm from his eyes. To read a book placed at a distance of 25 cm from him what type of spectacles should he use? What be the power of the spectacles?

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In this case a lens is to be used which will form the image of the objects situated at a distance of 25 cm at 15 cm from the man . i.e ., u=25 cm ,v=15 cm ,u and v are both positive .
From the equation `1/v-1/u=1/f` we have , `1/(15)-1/(25)=1/f or , 1/f=(5-3)/(75) or, f=(75)/2=37.5`cm
so the lens to be used is a concave lens of focal length 37.5 cm. Power of the lens,
`P=(-100)/f=(-100)/(75)xx2=(-8)/(3)=-2.67` dioptre.
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