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10th Class Science Light - Reflection and Refraction Question Bank Light

  • question_answer
    A concave lens of focal length\[f\]produces an image equal to \[\frac{1}{n}\] of the size of the object, the distance of the object from the lens is

    A)  \[(n-1)F\]            

    B)         \[(n+1)F\]

    C)  \[\left( \frac{n-1}{n} \right)F\] 

    D)         \[\left( \frac{n+1}{n} \right)F\]

    Correct Answer: A

    Solution :

                    \[m=\frac{I}{O}=\frac{v}{u}=\frac{1}{n}\] \[\Rightarrow \]               \[v=\frac{u}{n}\] Now,     \[\frac{1}{v}-\frac{1}{u}=\frac{1}{f}\] \[\Rightarrow \]               \[\frac{1}{f}=-\frac{1}{u}+\frac{n}{u}=\frac{(n-1)}{u}\] \[\therefore \]  \[u=(n-1)f\]


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