In a vacuum, electromagnetic waves travel at the speed of light, 
, which is 299,792,458 m/s. In other transparent materials, they travel at a reduced speed 
, according to the formula
, where 
is the refractive index of the medium.
Most natural electromagnetic waves are approximately plane waves. That is, each point is part of a planar wavefront, perpendicular to the direction of motion, on which the electric and magnetic field vectors are constant. For a plane electromagnetic wave, Maxwell's equations require that the electric and magnetic field vectors are perpendicular to each other, and to the direction of motion.
Usually the amplitude of the electric field vector at a point varies in a sinusoidal manner, and the wave is a simple harmonic wave. For simple harmonic electromagnetic waves, the electric vector can thus be described fully by giving the amplitude and phase for both its 
and 
-coordinates. Without loss of generality, you can assume that the x-coordinate has nonzero amplitude. The qualitative properties of such waves can thus be described by giving the phase difference and amplitude ratio for these components.
