Electromagnetic spectrum
The full range of frequencies (or equivalently wavelengths) of EM radiation, from kilohertz radio to zettahertz gamma rays. All regions are the same physical phenomenon — classical EM waves — differing only in ω.
Definition
The electromagnetic spectrum is the full range of frequencies at which electromagnetic waves can propagate. Conventional labels divide it into regions: radio (below ~300 GHz), microwaves (300 MHz – 300 GHz), infrared (~300 GHz – 430 THz), visible (430 – 750 THz, wavelengths 400–700 nm), ultraviolet (750 THz – 30 PHz), X-ray (30 PHz – 30 EHz), and gamma rays (above ~30 EHz). All regions are the same physical phenomenon — solutions of the same Maxwell wave equation, propagating at c in vacuum — differing only in wavelength λ = c/f.
The boundaries are historical and practical, not physical: the visible window (400–700 nm) is the range transmitted cleanly through Earth's atmosphere and to which evolution tuned vertebrate eyes; radio frequencies are low enough that classical antennas work; gamma rays are produced by nuclear transitions. Each region has distinctive technology: vacuum tubes and solid-state amplifiers for radio, magnetrons and klystrons for microwaves, thermal sources and semiconductor emitters for infrared, arc lamps and lasers for visible, synchrotrons and plasma discharges for UV–X-ray, and radioactive nuclei and cosmic sources for gamma. The unification via Maxwell's equations in 1865 (confirmed experimentally by Hertz's radio-wave detection in 1887) is one of the great syntheses of physics: radio and visible light had appeared to be completely different phenomena until the theory showed them to be the same wave at different frequencies.