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.