NEET Physics Photo Electric Effect, X- Rays & Matter Waves NEET PYQ-Photo Electric Effect,X-Rays

  • question_answer
    Light with an energy flux of \[25\times {{10}^{4}}W{{m}^{-2}}\] falls on a perfectly reflecting surface at normal incidence. If the surface area is \[15\,c{{m}^{2}},\] the average force exerted on the surface is              [NEET 2014]

    A)  \[1.25\times {{10}^{-6}}N\]

    B)       \[2.50\times {{10}^{-6}}N\]

    C)  \[1.20\times {{10}^{-6}}N\]    

    D)   \[3.0\times {{10}^{-6}}N\]

    Correct Answer: A

    Solution :

    Energy flux \[=\frac{25\times {{10}^{4}}\text{J}}{s{{m}^{2}}}\]
    Force on unit area = momentum transferred in unit time on unit area \[=\frac{25\times {{10}^{4}}}{C}\]
    \[=\frac{25\times {{10}^{4}}}{3\times {{10}^{8}}}\]
    \[=\,8.3\times {{10}^{-4}}N/{{m}^{2}}\]
    Force on the \[15\times {{10}^{-4}}{{m}^{2}}\] area
    \[=8.3\times {{10}^{-4}}N/{{m}^{2}}\times 15\times {{10}^{-4}}{{m}^{2}}\]
    \[=124.5\times {{10}^{-8}}N\]
    \[1.25\times {{10}^{-6}}N\]


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