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JIPMER 2005
Exam
JIPMER
Year
2005
Subject
Physics
Chapter
Ray Optics

JIPMER 2005 · Physics · Ray Optics

A person who can see things most clearly at a distance of 10 cm ,...

A person who can see things most clearly at a distance of 10 cm , requires spectacles to enable to see clearly things at a distance of 30 cm . What should be the focal length of the spectacles?

Options

  1. A15 cm (Concave)
  2. B15 cm (Convex)
  3. C10 cm
  4. D0

Correct answer

A. 15 cm (Concave)

Step-by-step solution

A concave lens is required by the person so that the lens forms the image of object distant 30 cm , at 10 cm it means u=−30cm,v=−10cm1f=1v−1u=1−10−1−30=−230 or 1f=−115 So f=−15cm\begin{aligned} u & =-30 \mathrm{ cm}, v=-10 \mathrm{ cm} \\ \frac{1}{f} & =\frac{1}{v}-\frac{1}{u} \\ & =\frac{1}{-10}-\frac{1}{-30}=-\frac{2}{30} \\ \text{ or } \quad \frac{1}{f} & =\frac{-1}{15} \\ \text{ So } \quad f & =-15 \mathrm{ cm}\end{aligned} Minus sign singnifies that lens used is concave
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