BHU PMT BHU PMT (Mains) Solved Paper-2009

  • question_answer
    The recoil speed of a hydrogen atom after it emits a photon in going from\[n=5\]state to\[n=1\]state is

    A) \[4.718\text{ }m{{s}^{-1}}\]        

    B) \[7.418\text{ }m{{s}^{-1}}\]

    C)  \[4.178\text{ }m{{s}^{-1}}\]      

    D)  \[7.148\text{ }m{{s}^{-1}}\]

    Correct Answer: C

    Solution :

                      \[E={{R}_{\infty }}hc\left[ 1-\frac{1}{25} \right]\]                 Momentum of photon emitted is                 \[P=\frac{E}{c}={{R}_{\infty }}h\left[ \frac{24}{25} \right]\]                 Recoil momentum of H-atom will also be p                 \[\Rightarrow \] \[mv=p\]                 \[\Rightarrow \]\[v=\frac{p}{m}=\frac{(1.097\times {{10}^{7}})(6.626\times {{10}^{-34}})24}{(25)(1.67\times {{10}^{-27}})}\]                 \[\Rightarrow \] \[v=4.178\,m{{s}^{-1}}\]


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