\(d\,\, = \,\,\frac{A}{{4\pi R}}\,\, = \,\,\frac{{\pi {r^2}}}{{4\pi R}}\,\, = \,\,\,\frac{{{r^2}}}{{4R}}\,\, = \,\,\frac{{{{\left( {20\,\, \times \,\,{{10}^{ - 3}}} \right)}^2}}}{{4\,\, \times \,\,1}}\)
\( = \,\,\frac{{400\,\, \times \,\,{{10}^{ - 6}}}}{4}\,\, = \,\,{10^{ - 4}}\,m\,\, = \,\,0.1\,\,mm\)