a
\({\lambda _0}\, = \,\,400\,\,nm\,\, = \,\,4000\,\,\mathop A\limits^ \circ  \,\,\,\,\,\,\)

\(K.E{._{\max .}}\, = \,\,\,\frac{{12400\,\,e\,V\,\,\mathop A\limits^ \circ  }}{\lambda }\,\, - \,\,\frac{{12400\,\,\,e\,V\,\,\mathop A\limits^ \circ  }}{{{\lambda _0}}}\)

\(1.5\,\,eV\,\, = \,\,\,\frac{{12400\,\,e\,V\,\,\mathop A\limits^ \circ  }}{\lambda }\,\, - \,\,\frac{{12400\,\,eV}}{{4000}}\,\,\, \Rightarrow \,\,1.5\,\,e\,V\,\)

\( = \,\,\frac{{12400\,\,eV\,\,\mathop A\limits^ \circ  }}{\lambda }\, - \,3.1\,\,e\,V\,\)

\((1.5\,\, + \,\,3.1)\,\,e\,V\,\, = \,\,\frac{{12400\,\,\,eV\,\,\mathop A\limits^ \circ  }}{\lambda }\,\,\,\, \Rightarrow \,\,\,\lambda \,\, = \,\,\,\frac{{12400\,\,\mathop A\limits^ \circ  }}{{4.6}}\)

\( \Rightarrow \,\,\,\lambda \,\, = \,\,2700\,\,\mathop A\limits^ \circ  \)