NEET Sample Paper NEET Sample Test Paper-12

  • question_answer
    Two identical metal plates show photoelectric effect by a light of wavelength\[{{\lambda }_{A}}\]falls on plate A and\[{{\lambda }_{B}}\]on plate\[B({{\lambda }_{A}}=2{{\lambda }_{B}}).\]The maximum kinetic energy is

    A) \[2{{K}_{A}}={{K}_{B}}\]                  

    B) \[{{K}_{A}}<{{K}_{B}}/2\]

    C) \[{{K}_{A}}=2{{K}_{B}}\]                  

    D) \[{{K}_{A}}={{K}_{B}}/2\]

    Correct Answer: B

    Solution :

     \[\frac{hc}{\lambda }={{W}_{0}}+{{K}_{\max }}\Rightarrow \frac{hc}{{{\lambda }_{A}}}={{W}_{0}}+{{K}_{A}}\]      (i) and \[\frac{hc}{{{\lambda }_{B}}}={{W}_{0}}+{{K}_{B}}\]                           (ii) Subtracting (i) from (ii), \[hc\left[ \frac{1}{{{\lambda }_{B}}}-\frac{1}{{{\lambda }_{A}}} \right]={{K}_{B}}-{{K}_{A}}\] \[\Rightarrow \]\[hc\left[ \frac{1}{{{\lambda }_{B}}}-\frac{1}{2{{\lambda }_{B}}} \right]={{K}_{B}}-{{K}_{A}}\] \[\Rightarrow \]\[\frac{hc}{2{{\lambda }_{B}}}={{K}_{B}}-{{K}_{A}}\]                         ?(iii) From (ii) and (iii), \[2{{K}_{B}}-2{{K}_{A}}={{W}_{0}}+{{K}_{B}}\] \[\Rightarrow \]\[{{K}_{B}}-2{{K}_{A}}={{W}_{0}}\] \[\Rightarrow \]\[{{K}_{A}}=\frac{{{K}_{B}}}{2}-\frac{{{W}_{0}}}{2}\]which gives \[{{K}_{A}}<\frac{{{K}_{B}}}{2}.\]


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