What focal length should the reading spectacles have for a person whose near point is `50cm` ? (NCERT Solved Example)
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The book to be read is at `u = - 25 cm` and the image is needed at `v = - 50 cm`. Therefore, focal length `(f)` of the required lens is given by `(1)/(f) = (1)/(v) - (1)/(u) = (1)/(-50) + (1)/(25) = (1)/(50)` `= 50 cm` (convex lens)
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