$\therefore \,\,\lambda \,\, = \,\,\frac{{{N_A}hc}}{E}\,\,\, = \,\,\frac{{(6.022\, \times \,{{10}^{23}})\,(6.626\, \times \,{{10}^{ - 34}})\,(3\, \times \,{{10}^8})}}{{242\, \times \,{{10}^3}}}$
$ = \,\,4.947\, \times \,{10^{ - 7}}\,m\,\,\, = \,494.7\, \times \,{10^{ - 7}}\,m\,\, = \,494.7\,nm\, \approx \,494\,nm$
( $m= 1.67 \times 10^{-27}\, kg, h = 6.63 \times 10^{-34}\, J\,s$)