Ta có ${{x}_{M}}=k\frac{\lambda D}{a}\Rightarrow \lambda =\frac{\text{a}\text{. }{{\text{x}}_{M}}}{kD}=\frac{2,2}{k}$ (với k bán nguyên) mà $0,4<\lambda <0,76\Rightarrow 2,9<k<5,5\Rightarrow k\in \left\{ 3,5;4,5 \right\}$ $\Rightarrow \lambda \in \left\{ 0,57;0,44 \right\}$