NEET Sample Paper NEET Sample Test Paper-50

  • question_answer 29) When a metallic surface is illuminated with monochromatic light of wavelength\[\lambda \], stopping potential for photoelectric current is \[3{{V}_{0}}\], When the same metallic surface is illuminated with a light of wavelength\[2\lambda \], the stopping potential is\[{{V}_{0}}\]. The threshold wavelength for the surface is

    A) \[6\lambda \]                 

    B)   \[4\lambda \]

    C) \[4\lambda /3\]              

    D) \[8\lambda \]

    Correct Answer: B

    Solution :

    Einstein?s photoelectric equation, \[\frac{hc}{\lambda }=e(3{{V}_{0}})+W\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,......(i)\] \[\frac{hc}{2\lambda }=e({{V}_{0}})+W\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,......(i)\] Solving these equations, we get; \[\frac{hc}{\lambda }=\frac{3hc}{2\lambda }+W-3W\] \[or\,\,\,\,\,\,2W=\frac{hc}{2\lambda }\,\,\,\,or\,\,\,\,\frac{hc}{{{\lambda }_{0}}}=\frac{hc}{4\lambda }\] So, \[{{\lambda }_{0}}\,=\,\,4\lambda \]

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