The diffraction pattern of a light of wavelength 400nm diffracting...

Width of ${1}^{st}$ secondary maxima $= \dfrac{\lambda}{a} \cdot D$

Here

$a = 0 . 2 \times {10}^{- 3}m$

$\lambda = 400 \times {10}^{- 9}m$

$D = 100 \times {10}^{- 2} = 1m$

Therefore, width of ${1}^{st}$ secondary maxima

$= \dfrac{400 \times {10}^{- 9}}{0 . 2 \times {10}^{- 3}} \times 1$

$= 2 \times {10}^{- 3}m = 2mm$