The electric field associated with an electromagnetic wave in vacuum is given by $\vec{E}$ = 40cos(kz − 6 × 10⁸ t)$\hat{i}$, where E, z and t are in volt per meter, meter and second respectively. The value of wave vector k is:

2 m^-1

Compare the given equation with $E=E_0 \cos (kz-\omega t)$

We get, $\omega=6 \times 10^8 s^{-1}$

Wave vector, $k=\frac{\omega}{c}=\frac{6 \times 10^8 s^{-1}}{3 \times 10^8 ms^{-1}}=2 m^{-1}$