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JEE Main & Advanced Physics Ray Optics Sample Paper Topic Test - Reflection of Light

  • question_answer
    The focal length of a concave mirror is 30 cm. Find the position of the object in front of the mirror, so that the image is three times the size of the object.

    A) 20 cm (only)

    B) 40 cm (only)

    C) 30 cm (only)

    D) 20 cm or 40 cm

    Correct Answer: D

    Solution :

    [d]
    Here image can be real or virtual. If the image is real
    \[f=-\,30,\,u=?,\,m=-3\]
    \[m=\frac{f}{f-u}\Rightarrow -\,3\,=\frac{-\,30}{-\,30\,-\,u};\,\,u=-\,40\,cm\]
    If the image is virtual
    \[m=\frac{f}{f-u}\Rightarrow 3=\frac{-\,30}{-\,30\,-\,u}\Rightarrow \,u=-\,20\,cm\]


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