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Uttarakhand PMT Uttarakhand PMT Solved Paper-2006

  • 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)  Zero

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


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