b
(b)Distance between \({n^{th}}\) Bright fringe and \({m^{th}}\) dark fringe \((n > m)\)
\(\Delta x = \left( {n - m + \frac{1}{2}} \right)\beta = \left( {5 - 3 + \frac{1}{2}} \right) \times \frac{{6.5 \times {{10}^{ - 7}} \times 1}}{{1 \times {{10}^{ - 3}}}}\)
=\(1.63\,mm\)