Q121 / 160 · JIPMER 2005 · Physics

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?

  • Correct answer
    A
    15 cm (Concave)
  • B
    15 cm (Convex)
  • C
    10 cm
  • D
    0

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 $\begin{aligned} u & =-30 \mathrm{~cm}, v=-10 \mathrm{~cm} \\ \frac{1}{f} & =\frac{1}{v}-\frac{1}{u} \\ & =\frac{1}{-10}-\frac{1}{-30}=-\frac{2}{30} \\ \text { or } \quad \frac{1}{f} & =\frac{-1}{15} \\ \text { So } \quad f & =-15 \mathrm{~cm}\end{aligned}$ Minus sign singnifies that lens used is concave

Hence, the correct answer is option: 'A'.

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