An electromagnetic wave (EM wave) is an oscillating electric field and an oscillating magnetic field that sustain each other and travel through space together. The electric field (E) and the magnetic field (B) are perpendicular to each other and perpendicular to the direction of propagation: an EM wave is a transverse wave.
Important properties that set EM waves apart from, for example, sound waves:
- No medium required. EM waves also propagate through a vacuum — which is why satellite communication works and sunlight reaches the earth. Sound needs air (or another medium); EM radiation does not.
- Speed of light. In a vacuum, and to a close approximation in air, all EM waves travel at c ≈ 3 × 108 m/s — from the longest radio wave to gamma radiation.
- Straight-line propagation. EM waves in principle travel in straight lines ("line of sight"), although low frequencies can bend around the earth and reflect off the ionosphere.
Radio waves, radar, infrared, visible light, ultraviolet, X-rays and gamma rays are all the same physical phenomenon; they differ only in frequency (and therefore wavelength). That single continuum is called the electromagnetic spectrum (EMS) — the working domain of the EW specialist.