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Punjab Medical Punjab - MET Solved Paper-2010

  • question_answer
    Minimum light intensity that can be perceived by normal human eye is about\[{{10}^{-10}}Wb\,\,{{m}^{-2}}\]. What is the minimum number of photons of wavelength 660 nm that must enter the pupil in\[1s\]for one to see the object? Area of cross-section of the pupil is\[{{10}^{-4}}{{m}^{2}}\].

    A) \[3.3\times {{10}^{2}}\]                

    B) \[3.3\times {{10}^{3}}\]

    C) \[3.3\times {{10}^{4}}\]                

    D) \[3.3\times {{10}^{5}}\]

    Correct Answer: C

    Solution :

    Intensity,\[I={{10}^{-10}}Wb{{m}^{-2}}={{10}^{-10}}J{{s}^{-1}}{{m}^{-2}}\]. Let the number of photons required be\[n\]. Then\[\frac{nhv}{{{10}^{-4}}}={{10}^{-10}}\] Hence,\[n={{10}^{-10}}\times {{10}^{-4}}/hv={{10}^{-14}}\frac{\lambda }{hc}\]                 \[=\frac{{{10}^{-14}}\times 660\times {{10}^{-9}}}{6.6\times {{10}^{-34}}\times 3\times {{10}^{8}}}=3.3\times {{10}^{4}}\]


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