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NEET Sample Paper NEET Sample Test Paper-86

  • 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] The person suffers from short-sightedness because he is unable to see objects at normal distance but can see objects at nearer distance.                         He will require concave lens. Things placed at 30 cm will appear to be placed at 10cm. So, u = 30; v = 10; f = ? From lens formula \[\frac{1}{v}-\frac{1}{u}=\frac{1}{f}\] \[\Rightarrow \] \[-\frac{1}{10}+\frac{1}{30}=\frac{1}{f}\] \[\Rightarrow \] \[\frac{-3+1}{30}=\frac{1}{f}\] \[\Rightarrow \] \[f=-\frac{30}{2}=-15cm\] Negative sign indicates that lens used is concave.


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