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A person can see distinctly up to a dis...

A person can see distinctly up to a distance of 2 m and no further. To see distinctly up to a long distance what type of spectacles should he use? What will be the power of the lens of the spectacles?

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As the person can see distinctly up to a distance of 2m , the defect of the eye is short sight, so to see distinctly up to a long distance he will have to use concave lens. The focal length of the lens will be such that the image of the objects at infinity will be formed at a distance of 2 m from the eye i.e ., u=`oo`,v=-2 m =-200cm (negative).
Suppose the focal length of the lens =f .
From the equation `1/v-1/u=1/f` we have ,
`1/(-200)-1/(00)=1/f or, f=-200 cm`.
`therefore` Power of the lens,
`P=(100)/f=(100)/(-200)=-0.5 m^(-1)`
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