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JEE Main & Advanced Physics Ray Optics Question Bank Photometry

  • question_answer
    In a movie hall, the distance between the projector and the screen is increased by 1% illumination on the screen is [CPMT 1990]

    A)            Increased by 1%                 

    B)            Decreased by 1%

    C)            Increased by 2%                 

    D)            Decreased by 2%

    Correct Answer: D

    Solution :

                       \[I=\frac{L}{{{r}^{2}}}\] \[\Rightarrow \ \ \frac{dI}{I}\ =-\frac{2dr}{r}\] (\[\because \] L = constant) Þ  \[\frac{dI}{I}\times 100\ =-\frac{2\times dr}{r}\times 100\] \[=-2\times 1=-2%\]


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