\(\therefore\) Distance between first two dark bands on each side of central maximum is the width of central maximum
\(=2 x=\frac{2 \lambda D}{d}\)
\(=\frac{2 \times 5.4 \times 10^{-7} \times 1.4}{8 \times 10^{-4}}\)
\(=1.89 \times 10^{-3} m =1.89 mm\)