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JEE Main & Advanced Physics Photo Electric Effect, X- Rays & Matter Waves Question Bank Critical Thinking

  • question_answer
    The minimum intensity of light to be detected by human eye is \[{{10}^{-10}}W/{{m}^{2}}\]. The number of photons of wavelength \[5.6\times {{10}^{-7}}m\] entering the eye, with pupil area \[{{10}^{-6}}{{m}^{2}}\], per second for vision will be nearly

    A)            100

    B)            200

    C)            300

    D)            400

    Correct Answer: C

    Solution :

                       By using \[I=\frac{P}{A};\]  where P = radiation power                    Þ \[P=I\times A\]Þ \[\frac{nh\,c}{t\lambda }=IA\]Þ \[\frac{n}{t}=\frac{IA\lambda }{hc}\]                    Hence number of photons entering per sec the eye \[\left( \frac{n}{t} \right)=\frac{{{10}^{-10}}\times {{10}^{-6}}\times 5.6\times {{10}^{-7}}}{6.6\times {{10}^{-34}}\times 3\times {{10}^{8}}}\]= 300.


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