The magnetic field in a plane electromagnetic wave travelling in glass ($n = 1\cdot 5$) is given by $B_y = (2 \times 10^{-7}\text{ T}) \sin (\alpha x + 1\cdot 5 \times 10^{11} t)$ where $x$ is in metres and $t$ is in seconds. The value of $\alpha$ is : (A) $0\cdot 5 \times 10^3\text{ m}^{-1}$ (B) $6\cdot 0 \times 10^2\text{ m}^{-1}$ (C) $7\cdot 5 \times 10^2\text{ m}^{-1}$ (D) $1\cdot 5 \times 10^3\text{ m}^{-1}$