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What focal length should the reading spectacles have for a person for whom the least distance of distinct vision is 50 cm?(a) + 25 cm(b) – 25 cm(c) + 50 cm(d) – 50 cmI had been asked this question in final exam.I would like to ask this question from Optical Instruments topic in division Ray Optics and Optical Instruments of Physics – Class 12

Answer»

Right choice is (C) + 50 CM

Best explanation: By thin LENS formula,

\(\frac {1}{f} = \frac {1}{v} – \frac {1}{U}\)

\(\frac {1}{f} = \frac {1}{(-50)} – \frac {1}{(-25)}\)

f = +50 cm.



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