JCECE Engineering JCECE Engineering Solved Paper-2009

  • question_answer
    If  \[{{\lambda }_{1}}\] and \[{{\lambda }_{2}}\] are the wavelengths of the first members of the Lyman and Paschen series respectively, then \[{{\lambda }_{1}}:{{\lambda }_{2}}\] is

    A) \[1:3\]                                  

    B) \[1:30\]

    C) \[7:50\]               

    D) \[7:108\]

    Correct Answer: D

    Solution :

    For first line of Lyman series,\[{{n}_{1}}=1\]and\[{{n}_{2}}=2\]. \[\therefore \]  \[\frac{1}{{{\lambda }_{1}}}=R\left( \frac{1}{{{1}^{2}}}-\frac{1}{{{2}^{2}}} \right)=R\left( 1-\frac{1}{4} \right)=\frac{3R}{4}\] For first line of Paschen series\[{{n}_{1}}=3\]and\[{{n}_{2}}=4\] \[\therefore \]  \[\frac{1}{{{\lambda }_{2}}}=R\left( \frac{1}{{{3}^{2}}}-\frac{1}{{{4}^{2}}} \right)=R\left( \frac{1}{9}-\frac{1}{16} \right)=\frac{7R}{144}\] \[\therefore \]  \[\frac{{{\lambda }_{1}}}{{{\lambda }_{2}}}=\frac{7R}{144}\times \frac{4}{3R}=\frac{7}{108}\]


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