English: Diagram of a portion of a linearly polarized plane electromagnetic wave showing the directions of the electric field (red arrows) and the magnetic field (green arrows) in space. In a plane wave the electric and magnetic field have a constant direction and magnitude at every point on a plane perpendicular to the direction of the wave, indicated by the small red and green arrows on the grey planes. The electric and magnetic field are perpendicular, and are perpendicular to the direction of the wave S. In a monochromatic, harmonic wave, shown here, the electric and magnetic field have a sinusoidal variation along the direction of motion. This is shown by the red and green bands on the ray R, which represent the length of the vectors at each point along the ray. The entire wave is moving in the direction S at the speed of light. λ is the wavelength of the wave.