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

  • question_answer
    A radiation of energy 'E' falls normally on a perfectly reflecting surface. The momentum on a transferred to the surface is (c = velocity of light)                                                                                                              [NEET  2015]

    A)  \[\frac{E}{c}\]             

    B)       \[\frac{2E}{c}\]

    C)  \[\frac{2E}{{{c}^{2}}}\]                    

    D)  \[\frac{E}{{{c}^{2}}}\]

    Correct Answer: B

    Solution :

    The radiation energy is given by
    \[E=\frac{hc}{\lambda }\]
    Initial momentum of the radiation is
    \[{{\mathbf{P}}_{i}}=\frac{h}{\lambda }=\frac{E}{c}\]
    The reflected momentum is
    \[{{\mathbf{P}}_{r}}=-\frac{h}{\lambda }=-\frac{E}{c}\]
    So, the change in momentum of light is
    \[\Delta {{\mathbf{P}}_{light}}={{\mathbf{P}}_{r}}-{{\mathbf{P}}_{i}}=-\frac{2E}{c}\]
    Thus, the momentum transferred to the surface is
    \[\Delta {{\mathbf{P}}_{light}}=\frac{2E}{c}\]


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