\({E_K} - {E_L}\)
\( = \,\,\frac{{hc}}{{{\lambda _{K\alpha }}}}\, = \,\,\frac{{6.6\, \times \,\,{{10}^{ - 34}} \times \,\,3\,\, \times \,\,{{10}^8}}}{{0.021\,\, \times \,\,{{10}^{ - 9}}}}\)
\( = \,\,9.46\,\, \times \,\,{10^{ - 15\,}}J\,\, = \,\,59\,\,keV\)