b
\({\text{x}}\,=\,\frac{{\lambda {\text{D}}}}{d}\,\, = \,\,3\,\, \times \,\,\left( {5000\,\, \times \,\,{{10}^{ - 10}}} \right)\,\, \times \,\,\frac{2}{{0.2\,\, \times \,\,{{10}^{ - 3}}}}\)
\(\,\, = \,\,1.5\, \times \,\,{10^{ - 2}}m\,\, = \,\,1.5\,\,cm\)