Here
\( a=0.2 \times 10^{-3} \mathrm{~m} \)
\( \lambda=400 \times 10^{-9} \mathrm{~m} \)
\( D=100 \times 10^{-2}\)
Width of \(1^{\text {st }}\) secondary maxima
\( =\frac{400 \times 10^{-9}}{0.2 \times 10^{-3}} \times 100 \times 10^{-2} \)
\( =2 \mathrm{~mm}\)