JEE Main & Advanced Sample Paper JEE Main Sample Paper-14

  • question_answer
    Photoelectric emission is observed from a surface for frequencies \[{{v}_{1}}\] and\[{{v}_{2}}\]of the incident radiation\[({{v}_{1}}>{{v}_{2}})\]. If the maximum kinetic energies of the photoelectrons in the two cases are in the ratio 1 : k then the threshold frequency \[{{v}_{0}}\] is given by

    A)  \[\frac{{{v}_{2}}-{{v}_{1}}}{k-1}\]                            

    B)  \[\frac{k{{v}_{1}}-{{v}_{2}}}{k-1}\]

    C)  \[\frac{k{{v}_{2}}-{{v}_{1}}}{k-1}\]                         

    D)  \[\frac{{{v}_{2}}-{{v}_{1}}}{k}\]

    Correct Answer: B

    Solution :

     When frequency is \[{{v}_{1}}\], \[h{{v}_{1}}=h{{v}_{0}}+\frac{1}{2}\,mu_{1}^{2}\] When frequency is \[{{v}_{2}},\] \[h{{v}_{2}}=h{{v}_{0}}+\frac{1}{2}mu_{2}^{2}\] \[\because \]     \[\frac{1}{2}\,mu_{1}^{2}=\frac{1}{k}\,\left( \frac{1}{2}\,mu_{2}^{2} \right)\] \[\therefore \] from Eq.  (i) \[h{{v}_{1}}=h{{v}_{0}}+\frac{1}{2k}mu_{2}^{2}\] or            \[\frac{1}{2}\,mu_{2}^{2}=kh{{v}_{1}}-kh{{v}_{0}}\] From Eqs. (ii) and (iv) \[h{{v}_{2}}=h{{v}_{0}}+kh{{v}_{1}}-kh{{v}_{0}}\] or \[{{v}_{0}}\,(1-k)={{v}_{2}}-k{{v}_{1}}\] or            \[{{v}_{0}}=\frac{k{{v}_{1}}-{{v}_{2}}}{k-1}\]


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