The electric field of an electromagnetic wave travelling through vacuum is given by the equation E₀ = E sin(kx − ωt) .The quantity that is independent of wavelength is:

$\frac{k}{ω}$

Here, $k=\frac{2 \pi}{\lambda}, \omega=2 \pi v$

∴  $\frac{k}{\omega}=\frac{2 \pi / \lambda}{2 \pi v}=\frac{1}{v \lambda}=\frac{1}{c}$               $(∵ c=v \lambda)$

Where c is the speed of electromagnetic wave in vacuum. It is a constant whose value is 3 × 10⁸ ms^-1.