\(\,\,\beta \,\, = \,\,\frac{{1.92\,\, \times \,\,{{10}^{ - 2}}}}{{20}}\,\, = \,\,96\,\, \times \,\,{10^{ - 5}}\,\,m\)
\(\because \,\,\beta \,\, = \,\,\frac{{\lambda D}}{d}\,\,\,\,\,\,\therefore \,\,\,\lambda \,\, = \,\,\,\frac{{\beta d}}{D}\,\,\,\, = \,\,\,\frac{{96\,\, \times \,\,{{10}^{ - 5}} \times \,\,75\,\,\, \times \,\,{{10}^{ - 5}}}}{{1.2}}\,\)
\( = \,\,80\,\, \times \,\,75\,\, \times \,\,{10^{ - 10}} = \,\,\,6000\,\,\mathop A\limits^ \circ \)