Q.The far point of a myopic person is 70 cm in front of the eye. Calculate the focal length and power of the lens needed by him to see the distant objects clearly.
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Start your 14-day free trial to unlock the full solution →A myopic (short-sighted) eye's far point is nearer than infinity; a diverging lens of focal length equal (in magnitude) to that far-point distance forms a virtual image of a distant object exactly at the far point, so it can be seen clearly.
A person suffering from myopia (near-sightedness) cannot see distant objects clearly because their eye's far point is at a finite distance (here, 70 cm) instead of infinity. To correct this, a lens is needed that takes an object at infinity and forms its image at the eye's far point (70 cm in front of the eye), so the eye can focus on that image as if it were the object.
Using the lens formula 1/v - 1/u = 1/f with:
u = -infinity (object at infinity, sign convention: distances measured from the lens, incident light taken as positive x-direction),
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