Width of $1^{\text {st }}$ secondary maxima $=\frac{\lambda}{a} \cdot D$ Here $$ \begin{aligned} & a=0.2 \times 10^{-3} \mathrm{~m} \\ & \lambda=400 \times 10^{-9} \mathrm{~m} \\ & D=100 \times 10^{-2} \end{aligned} $$ Width of $1^{\text {st }}$ secondary maxima $$ \begin{aligned} & =\frac{400 \times 10^{-9}}{0.2 \times 10^{-3}} \times 100 \times 10^{-2} \\ & =2 \mathrm{~mm} \end{aligned} $$