a
\({x_n} = \,\,\frac{{n\,{f}\lambda }}{a}\,\, \Rightarrow \,\,\,\lambda \,\, = \,\,\frac{{a{x_n}}}{{{f}n}}\,\, = \,\,\,\frac{{3\,\, \times \,\,{{10}^{ - 4}} \times \,\,5\,\,\, \times \,\,{{10}^{ - 3}}}}{{3\,\, \times \,\,1}}\,\)
\( = \,\,\,5000\,\,\mathop A\limits^ \circ \,\,\,\,\,[\,\because \,\,\,n\,\, = \,\,3]\)