KVPY Sample Paper KVPY Stream-SX Model Paper-2

  • question_answer
    An electron in the ground state of hydrogen was excited to a higher energy level using  monochromatic radiations of wave length \[(\lambda )975\]\[\overset{{}^\circ }{\mathop{\text{A}}}\,\]. The longest wave length that appears in the resulting spectrum is due to transition from:

    A) \[{{n}_{4}}\xrightarrow{{}}{{n}_{1}}\]            

    B) \[{{n}_{4}}\xrightarrow{{}}{{n}_{3}}\]

    C) \[{{n}_{5}}\xrightarrow{{}}{{n}_{4}}\]            

    D) \[{{n}_{5}}\xrightarrow{{}}{{n}_{1}}\]

    Correct Answer: B

    Solution :

    [b]
    The energy associated with radiation of wave length \[975\text{ }\overset{o}{\mathop{\Alpha }}\,\]is given by \[E=\frac{12400}{975}eV=12.72eV\]
    [for a radiation of wavelength x\[\overset{o}{\mathop{A}}\,\], we know\[E=\frac{12400}{x}eV\]]
    \[E=13.6\left[ \frac{1}{n_{1}^{2}}-\frac{1}{n_{2}^{2}} \right]eV\]
    Or \[E=13.6\left[ \frac{1}{1}-\frac{1}{n_{2}^{2}} \right]eV\left[ \because {{n}_{1}}=1 \right]\]
    Or \[12.72=13.6\left[ 1-\frac{1}{n_{2}^{2}} \right]\left[ \because E=12.72eV \right]\] 
    Or \[1-\frac{1}{n_{2}^{2}}=\frac{12.72}{13.6}\]
    Or \[\frac{1}{n_{2}^{2}}=\frac{13.6-12.72}{13.6}=\frac{0.88}{13.6}=\frac{1}{16}\]
    Or \[n_{2}^{2}=16\,\,or\,\,{{n}_{2}}=4\]  
    The transition \[{{n}_{4}}\xrightarrow{{}}{{n}_{3}}\]will give the longest wave length.


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