The electric field part of an electromagnetic wave in a medium is represented by $E_{x}=0$; $E_{y}=2.5 NC^{-1} \times \cos \left[\left(2 \pi \times 10^6 rads^{-1}\right) t-\left(\pi \times 10^{-2} radm^{-1}\right) x\right] ; E_{z}=0$. The wave is:

moving along x – direction with frequency 10⁶ Hz and wavelength 200 m

Given, $E_x=0$

$E_{y}=2.5 NC^{-1} \times \cos \left[\left(2 \pi \times 10^6 rads^{-1}\right) t-\left(\pi \times 10^{-2} radm^{-1}\right) x\right],  E_{z}=0$. This shows that the wave is propagating along x – axis. Comparing the given equation with $E=E_0 \cos (\omega t - kx)$, we have $\omega=2 \pi \times 10^6$

or $2 \pi v=2 \pi \times 10^6 \Rightarrow v=10^6 Hz$

and $\frac{2 \pi}{\lambda}=k=\pi \times 10^{-2} \Rightarrow \lambda=\frac{2 \pi}{\pi \times 10^{-2}}$ = 200 m