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RAJASTHAN PMT Rajasthan - PMT Solved Paper-2010

  • question_answer
    An electric bulb illuminates a plane surface. The intensity of illumination on the surface at the point 2m away from the bulb is \[5\times {{10}^{-4}}\] phot (lumen/\[c{{m}^{2}}\]). The line joining the bulb to the paint makes an angle of \[60{}^\circ \] with the normal to the surface. The intensity of the bulb in candela is

    A)  \[40\sqrt{3}\]                  

    B)         40

    C)  20                         

    D)         \[40\times {{10}^{-7}}\]

    Correct Answer: B

    Solution :

    Intensity of bulb                 \[I=\frac{L\cos \theta }{{{r}^{2}}}\]                 \[L=\frac{I\times {{r}^{2}}}{\cos \theta }=\frac{5\times {{10}^{-4}}\times {{(2\times {{10}^{2}})}^{2}}}{\cos {{60}^{o}}}\]                 \[L=40\,\,cd\]


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