A topic in the Open Knowledge Graph — a free, open map of 15,290 topics and the order to learn them in.
All electromagnetic waves travel at c = 3 × 10⁸ m/s in vacuum, described by c = fλ. The electromagnetic spectrum spans from radio waves (λ ~ km, f ~ kHz) through microwaves, infrared, visible light (400–700 nm), ultraviolet, X-rays, to gamma rays (λ ~ pm, f ~ EHz). Higher frequency means shorter wavelength and greater photon energy (E = hf). Visible light is only a tiny window of this spectrum, with different wavelengths perceived as different colors.
Map out the full spectrum on a log scale, placing familiar examples at each band (FM radio at 100 MHz, microwave oven at 2.45 GHz, green light at 550 nm, chest X-ray at 0.01 nm). Compute wavelengths and frequencies from c = fλ to build quantitative intuition.
The electromagnetic spectrum is not a collection of fundamentally different things — it is a single family of waves, all described by the same physics, differing only in frequency (and therefore wavelength and energy). Radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays are all oscillating electric and magnetic fields propagating through space. What makes them different is how fast those fields oscillate.
The governing relationship is c = fλ, where c = 3 × 10⁸ m/s is the speed of light in vacuum, f is frequency in Hz, and λ is wavelength in meters. Since c is fixed, frequency and wavelength are inversely related: doubling the frequency halves the wavelength. Radio waves might have wavelengths of kilometers and frequencies of kilohertz; gamma rays have wavelengths smaller than an atom (picometers) and frequencies exceeding 10²⁰ Hz. These are not approximations — c is a physical constant, exact by definition in SI units.
Energy enters through quantum mechanics: the energy of a single photon is E = hf, where h = 6.626 × 10⁻³⁴ J·s is Planck's constant. Higher frequency means higher photon energy. This is why UV radiation causes sunburn but radio waves do not — a UV photon carries enough energy to break chemical bonds in DNA, while a radio photon does not. It is also why X-rays and gamma rays are ionizing radiation: their photons can eject electrons from atoms. The energy difference between the top and bottom of the spectrum spans about 15 orders of magnitude.
A critical misconception to correct: all EM waves travel at exactly c in vacuum, regardless of frequency. X-rays do not travel faster than radio waves. The confusion often arises because higher-energy waves are more penetrating, which sounds like faster. But penetration depth is about interaction with matter, not propagation speed. Speed differences do appear in material media — glass slows different wavelengths differently, which is why prisms split white light into a rainbow (dispersion) — but in vacuum, c is universal.
The visible portion of the spectrum — roughly 400 nm (violet) to 700 nm (red) — is a tiny sliver, spanning less than one octave in frequency out of the ~45 octaves of the full spectrum. Our eyes evolved sensitivity to this window because it matches the peak emission of the Sun. Instruments extend our perception across the full spectrum: radio telescopes, infrared cameras, UV detectors, and X-ray imagers all reveal structure invisible to the naked eye, each probing matter's interaction with a different frequency range.