Question

Every series of hydrogen spectrum has an upper and lower limit in wavelength. The spectral series which has an upper limit of wavelength equal to $18752\, \mathring{A}$, is

• A. Balmer series
• B. Lyman series
• C. Paschan series
• D. Pound series

Answer 1

Answer: (C)

For hydrogen spectrum from Bohr's theory
$\frac{1}{\lambda} =R\left(\frac{1}{n_{1}^{2}}-\frac{1}{n_{2}^{2}}\right)$
$\left(\frac{1}{n_{1}^{2}}-\frac{1}{n_{2}^{2}}\right) =\frac{1}{\lambda R}$
$=\frac{1}{18752 \times 10^{-10} \times 1.097 \times 10^{7}}$
$=\frac{486}{10000}=\frac{7}{144}$
$=\frac{4^{2}-3^{2}}{3^{2} \times 4^{2}}$
$=\left(\frac{1}{3^{2}}-\frac{1}{4^{2}}\right)$
$\Rightarrow n_{1}=3,\, n_{2}=4$
It is true for Paschan series.