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Question
A nearsighted person cannot see beyond 25 cm. Assuming that the separation of the glass from the eye is 1 cm, find the power of lens needed to see distant objects.
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Solution
Given:
As the person wears spectacles at a distance of 1 cm from the eyes.
Distance of the image from the glass, v = Distance of the image from the eye − Separation between the glass and the eye
v = 25 − 1 = 24 cm = 0.24 m
For the near-sighted person, u = ∞.
The lens formula is given by
`1/v -1/u = 1/f`
On substituting the values in the above formula, we get:
`1/f = 1/((-0.24))-1/∞ = -42`
Power of the lens, P = `1/f` = − 4.2 D
Thus, the power of the lens required to see distant objects is − 4.2 D.
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