Ordering the Electromagnetic Spectrum by Energy

Every electromagnetic wave — from the radio signal reaching your phone to the gamma ray fired out of a decaying nucleus — is the same thing at heart: a self-sustaining oscillation of electric and magnetic fields propagating through space. In a vacuum, they all travel at exactly c = 3.00 × 10⁸ m/s, the speed of light. That speed never changes. What changes is frequency (f), which tells you how many wave crests pass a point per second, measured in hertz (Hz). Because speed is fixed, frequency and wavelength (λ) are inversely linked by c = fλ. Crank up the frequency and the wavelength shrinks proportionally. Halve the frequency and the wavelength doubles. There is no exception anywhere on the spectrum. Now here is the quantum twist that makes frequency so important: each electromagnetic wave is also a stream of photons, and the energy of one photon is given by E = hf, where h = 6.626 × 10⁻³⁴ J·s is Planck's constant. Higher frequency means higher photon energy — directly, linearly. This is why an X-ray can ionize (remove electrons from) an atom, while a radio-wave photon passes through you without effect. The spectrum is traditionally divided into seven regions, listed here from lowest to highest frequency (and therefore lowest to highest energy): Radio → Microwave → Infrared → Visible → Ultraviolet → X-ray → Gamma ray. A useful memory anchor: 'Really Mighty Instruments View Ultra-eXquisite Galaxies.' Visible light — the narrow band your eyes detect — spans roughly 400 nm (violet) to 700 nm (red). Everything below 400 nm in wavelength is higher energy than violet; everything above 700 nm is lower energy than red. Gamma rays, produced in nuclear reactions, carry photon energies above ~100 keV. Radio waves, used in broadcasts, carry photon energies below ~10⁻⁶ eV — roughly 100 billion times less energetic than a minimum gamma-ray photon, yet both travel at c. A common misconception: students sometimes think higher-frequency waves travel faster. They do not. Speed in vacuum is always c. Frequency only determines energy and wavelength, never propagation speed.