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BHU PMT BHU PMT (Mains) Solved Paper-2007

  • question_answer
    Light of wavelength \[\lambda =4000\overset{\text{o}}{\mathop{\text{A}}}\,\] and intensity \[100\text{ }W/{{m}^{2}}\]is incident on a plate of threshold frequency\[5.5\times {{10}^{14}}Hz\]. Find the number of photons incident per\[{{m}^{2}}\]per sec.

    A) \[{{10}^{21}}\]                                  

    B)  \[3.0\times {{10}^{19}}\]

    C)  \[2.02\times {{10}^{20}}\]          

    D) \[2.02\times {{10}^{21}}\]

    Correct Answer: C

    Solution :

                     Let number of photons emitted per sec be n. Then, intensity \[=100\text{ }W/{{m}^{2}}\] \[\Rightarrow \]               \[\frac{nhc}{\lambda }=100\] \[\Rightarrow \]               \[n=\frac{100\times \lambda }{hc}\]                 \[=\frac{10\times 4000\times {{10}^{-10}}}{6.6\times {{10}^{-34}}\times 3\times {{10}^{8}}}\]                 \[=2.02\times {{10}^{20}}\]


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