$\frac{1}{\lambda }\,\, = \,\,R{Z^2}\left[ {\frac{1}{{{{(4)}^2}}} - \frac{1}{{{{(7)}^2}}}} \right]\,\,\,\, \Rightarrow \,\,\,\,\frac{1}{\lambda }\,\, = \,\,R\left[ {\frac{1}{{16}} - \frac{1}{{49}}} \right]\,\,\,\, \Rightarrow \,\,\,\,\frac{1}{\lambda }\,\, = \,\,R\left[ {\frac{{49 - 16}}{{16 \times 49}}} \right]\,\,\,\,\,\,$

$\frac{1}{\lambda }\,\, = \,\,R\frac{{33}}{{784}}\,\, \Rightarrow \,\,\,\frac{1}{\lambda }\,\, = \,\,\frac{{33R}}{{784}}$ જેથી

 $\lambda \,\, = \,\,\frac{{784}}{{33R}}$