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JEE Main & Advanced Physics Ray Optics Question Bank Photometry

  • question_answer
    A lamp is hanging at a height 40 cm from the centre of a table. If its height is increased by 10 cm the illuminance on the table will decrease by 

    A)            10 %                                         

    B)            20%

    C)            27%                                          

    D)            36%

    Correct Answer: D

    Solution :

                       \[{{I}_{1}}=\frac{L}{r_{1}^{2}}=\frac{L}{1600}\] and \[{{I}_{2}}=\frac{L}{2500}\]                    \ % decease in illuminance            \[=\frac{{{I}_{1}}-{{I}_{2}}}{{{I}_{1}}}\times 100\]\[=\left( 1-\frac{1600}{2500} \right)\times 100\]\[=\frac{900}{2500}\times 100=36\]


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