Question

The shortest wavelength present in the Paschen series of spectral lines is

• A. 720 nm
• B. 790 nm
• C. 800 nm
• D. 820 nm

Answer 1

Answer: (D)

820 nm

Answer 2

The wavelength of the Paschen series

$\frac{1}{\lambda} = R\left\lbrack \frac{1}{3^{2}} - \frac{1}{n^{2}} \right\rbrack$

For shortest wavelength $n = \infty$

$\therefore\frac{1}{\lambda} = R\left\lbrack \frac{1}{9} - \frac{1}{\infty^{2}} \right\rbrack = \frac{R}{9}$

$\lambda = \frac{9}{R} = \frac{9}{1.097 \times 10^{7}}$

$= 8.20 \times 10^{- 7}m = 820nm.$