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Consider a rectangular coordinate system $(x,y,z)$ with unit vectors $a_{x}\:a_{y}$ and $a_{z}$. A plane wave traveling in the region $z\geq 0$ with electric field vector $E=10\cos\left ( 2\times 10^{8}t\:+\:\beta z\right )a_{y}$ is incident normally on the plane at $z=0$, where $\beta$ is the phase constant. The region $z\geq 0$ is in free space and the region $z<0$ is filled with a lossless medium (permittivity $\varepsilon \:=\:\varepsilon _{0}$, permeability $\mu \:=\:4\mu _{0}$, where $\varepsilon _{0}\:=\:8.85\times 10^{-12}\:\text{F/m}$ and $\mu _{0}\:=\:4\pi \times 10^{-7}\:\text{H/m}$). The value of the reflection coefficient is

- $\frac{1}{3}$
- $\frac{3}{5}$
- $\frac{2}{5}$
- $\frac{2}{3}$