\(\sqrt{ D ^2+ a ^2}- D =\frac{\lambda}{2}\)
\(D \left(1+\frac{ a ^2}{ D ^2}\right)^{1 / 2}- D =\frac{\lambda}{2}\)
\(D \left(1+\frac{1}{2} \times \frac{ a ^2}{ D ^2}\right)- D =\frac{\lambda}{2}\)
\(\frac{ a ^2}{2 D }=\frac{\lambda}{2} \Rightarrow a =\sqrt{\lambda . D }\)
\(=\sqrt{800 \times 10^{-6} \times 50}\)
\(a =0.2\,mm\)