A uniform plane wave with electric field $\vec{E}(x)=A_{y} \hat{a}_{y} e^{-j \frac{2 \pi x}{3}} \mathrm{~V} / \mathrm{m}$ is travelling in the air (relative permittivity, $\epsilon_{r}=1$ and relative permeability, $\mu_{r}=1$ ) in the $+x$ direction ( $A_{y}$ is a positive constant, $\hat{a}_{y}$ is the unit vector along the $y$ axis). It is incident normally on an ideal electric conductor (conductivity, $\sigma=\infty$ ) at $x=0$. The position of the first null of the total magnetic field in the air (measured from $x=0$, in metres) is $\_\_\_\_\_\_\_$.
- $-\frac{3}{4}$
- $-\frac{3}{2}$
- $-6$
- $-3$