Electromagnetic Spectrum
The electromagnetic spectrum is the full range of light waves, from long radio waves through microwaves, infrared, visible, UV, X-rays, and gamma rays.
Definition
The electromagnetic spectrum is the whole family of light waves, including kinds the eye cannot see: radio, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays are all the same kind of wave with different wavelengths, arranged from longest to shortest, and visible light is only a tiny slice of the whole spectrum. Every part of it travels at the speed of light in a vacuum, $3.00 \times 10^8\ \text{m/s}$, and needs no medium, with shorter wavelength meaning higher frequency and more energy per wave. Electromagnetic waves are coupled oscillating electric and magnetic fields predicted by Maxwell's equations, propagating at $c = 1/\sqrt{\mu_0\varepsilon_0}$; the spectrum is continuous, and its regional names are conventions: radio ($\lambda > 1\ \text{mm}$), microwave ($1\ \text{mm}$ to $1\ \text{m}$), infrared ($700\ \text{nm}$ to $1\ \text{mm}$), visible ($400$ to $700\ \text{nm}$), ultraviolet ($10$ to $400\ \text{nm}$), X-rays ($0.01$ to $10\ \text{nm}$), and gamma rays shorter still, with photon energy $E = hf = hc/\lambda$, and radiation above roughly $10\ \text{eV}$ becoming ionizing.
Example
A radio picks up radio waves, a microwave oven cooks with microwaves, a remote control uses infrared, and a doctor images bones with X-rays; FM radio at $100\ \text{MHz}$ has $\lambda = 3\ \text{m}$, while visible green light at $5.5 \times 10^{14}\ \text{Hz}$ has $\lambda = 3.00 \times 10^8 / 5.5 \times 10^{14} \approx 5.5 \times 10^{-7}\ \text{m}$, or $550\ \text{nm}$. That green photon carries $E = hc/\lambda \approx 3.6 \times 10^{-19}\ \text{J} \approx 2.3\ \text{eV}$, while a $0.1\ \text{nm}$ medical X-ray photon carries about $12{,}400\ \text{eV}$, some $5{,}500$ times more, enough to strip electrons from atoms.
Key Insight
Sunburn comes from ultraviolet, not from heat, since UV carries enough energy per wave to damage skin cells while infrared, which carries most of the warmth, does not. Every region of the spectrum is a different window on the universe: radio reveals cold gas, infrared sees through dust, visible shows stars, X-rays expose black-hole accretion, and gamma rays trace the most violent events, so astronomy became multi-messenger long before it left the electromagnetic spectrum.