What is the colour corresponding to the wavelength of light emitted when the electron in a hydrogen atom undergoes transition from $$n = 4$$  to $$n = 2?$$

Solution :
$$\eqalign{ & \frac{1}{\lambda } = R\left[ {\frac{1}{{n_1^2}} - \frac{1}{{n_2^2}}} \right] \cr & \,\,\,\,\,\, = 109677\left[ {\frac{1}{{{2^2}}} - \frac{1}{{{4^2}}}} \right]c{m^{ - 1}} \cr & \,\,\,\,\,\, = 20564.4\,\,c{m^{ - 1}} \cr & \lambda = \frac{I}{{20564.4\,\,c{m^{ - 1}}}} \cr & \,\,\,\,\, = 486 \times {10^{ - 7}}\,cm\,\,\,{\text{or}}\,\,\,486 \times {10^{ - 9}}\,m \cr} $$
Colour corresponding to this wavelength is blue.