The electromagnetic spectrum is the complete range of electromagnetic waves, ordered by wavelength (or equivalently frequency), from long-wavelength radio waves to ultra-short gamma rays. Every part of the spectrum is the same kind of wave — they all travel at $c$ in vacuum and obey $c=f\lambda$ — and they differ only in wavelength and frequency. As wavelength shrinks, frequency rises and so does the energy of each photon ($E=hf$), which is why X-rays and gamma rays are far more penetrating and dangerous than radio waves.
The relation $c=f\lambda$ ties the whole spectrum together: a higher frequency always means a shorter wavelength. There are no sharp boundaries between the bands; they overlap and are named by how they are produced and used. Going from the longest to the shortest wavelength:
- Radio waves ($\lambda>0.1\ \text{m}$, $f<10^{9}\ \text{Hz}$): produced by oscillating currents in antennas (accelerating charges in conductors). Used for radio and television broadcasting, and in mobile and satellite communication. AM/FM radio and TV signals are radio waves.
- Microwaves ($\lambda\sim1\ \text{mm}$ to $0.1\ \text{m}$, $f\sim10^{9}$–$10^{11}\ \text{Hz}$): produced by special vacuum tubes such as klystrons and magnetrons. Used in RADAR, satellite and mobile communication, and in microwave ovens (they make water molecules in food vibrate).
- Infrared (IR) ($\lambda\sim700\ \text{nm}$ to $1\ \text{mm}$): produced by hot bodies and vibrating molecules. We feel it as heat. Used in night-vision devices, TV remote controls, physiotherapy, and weather and Earth-observation satellites. IR is sometimes called heat radiation.
- Visible light ($\lambda\sim400$–$700\ \text{nm}$): the only part our eyes can detect, produced by atoms and molecules during electron transitions and by hot objects. It runs from violet (shortest) to red (longest) and lets us see the world.
- Ultraviolet (UV) ($\lambda\sim10$–$400\ \text{nm}$): produced by very hot bodies such as the Sun and by special UV lamps. It causes sunburn and is used to sterilise surgical instruments and water. The ozone layer absorbs most solar UV, protecting life.
- X-rays ($\lambda\sim0.01$–$10\ \text{nm}$): produced when high-speed electrons are suddenly stopped by a metal target. They penetrate soft tissue and are used in medical imaging and in studying crystal structure (X-ray diffraction).
- Gamma rays ($\lambda<0.01\ \text{nm}$, highest frequency): emitted by radioactive nuclei and in nuclear reactions. They are the most penetrating and energetic, used to treat cancer (radiotherapy) and to sterilise medical equipment.
A memory aid. In order of increasing frequency (decreasing wavelength): Radio, Microwave, Infrared, Visible, Ultraviolet, X-ray, Gamma. A common mnemonic is Roman Men Invented Very Unusual X-ray Guns. All these waves share the same speed $c$ in vacuum — only their wavelengths and frequencies (and hence photon energies and uses) differ.