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NEET Sample Paper NEET Sample Test Paper-30

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

    A)  15cm (concave)   

    B)  15cm (convex)

    C)  20cm            

    D)  40cm

    Correct Answer: A

    Solution :

    Since the person is not able to see distant object beyond 10cm hence, he should use concave lens in his spectacles. Concave lens will help to form the image of object distant 30cm i.e., when, \[u=-30cm,\] \[\upsilon =-10cm\] From formula, \[\Rightarrow \]            \[\frac{1}{f}=\frac{1}{\upsilon }-\frac{1}{u}\] \[\Rightarrow \]            \[\frac{1}{f}=\frac{1}{-10}-\frac{1}{-30}=-\frac{2}{30}\] \[\Rightarrow \]            \[\frac{1}{f}=\frac{1}{15}\] Hence,           \[f=-15cm.\] Here minus sign signifies that lens used is in concave.


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