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JAMIA MILLIA ISLAMIA Jamia Millia Islamia Solved Paper-2012

  • question_answer
        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?

    A)  15 cm (concave)             

    B)  15 cm (convex)

    C)  10 cm                                  

    D)  0

    Correct Answer: A

    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 \[u=-30\text{ }cm,\text{ }v=-10\text{ }cm\]                 \[\frac{1}{f}=\frac{1}{v}-\frac{1}{u}\]                 \[=\frac{1}{-10}-\frac{1}{-30}\]                 \[=-\frac{2}{30}\] Or           \[\frac{1}{f}=\frac{-1}{15}\] So,         \[f=-15cm\] Minus sign signifies that lens used is concave.


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