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A long -sighted man can clearly see at a...

A long -sighted man can clearly see at any distance beyond 2.5 m . What kind of lens in his spectacles does require to read books placed 25 cm from his eyes?

Text Solution

Verified by Experts

Let the focal length of the spectacles be f. Now, if an object is placed at 25 cm from the eye then virtual image will be created at a distance 250 cm from the eye .
Hence, u=-25 cm ,v =-2.5m =-250 cm
From the equation `1/v-1/u=1/f` , we get
`(-1)/(-250)-1/(-25)=1/f`
or, f=`(250)/9=27.78 cm `
So that should use convex lens of focal length 27.78 cm.
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