electromagnetic spectrum
/ ih-lek-troh-mag-NET-ik SPEK-truhm /
Think of a piano keyboard that runs not from low notes to high notes but from radio waves all the way to gamma rays. Visible light — the reds, greens and blues your eye can see — is just a few keys near the middle. The electromagnetic spectrum is that whole keyboard: the full range of light-like radiation, sorted from the longest, lowest-energy waves to the shortest, most energetic ones.
Physically, all of it is the same thing — electromagnetic radiation, oscillating electric and magnetic fields travelling at the speed of light — differing only in wavelength and frequency. From long to short you pass through radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays. Shorter wavelength means higher frequency and higher energy per photon, because a photon's energy is proportional to its frequency.
This matters for spectroscopy because different regions carry just the right amount of energy to stir up different kinds of molecular motion: microwaves nudge molecules into spinning faster, infrared sets bonds vibrating, and visible or ultraviolet light lifts electrons to higher orbitals. A spectroscopist chooses the region of the spectrum that matches the question being asked, which is why there are so many named techniques.
A microwave oven heats food using radiation a few centimetres long; a TV remote signals with infrared a touch longer than red light; a hospital X-ray uses waves thousands of times shorter still. All three are the very same kind of radiation, separated only by where they sit on the spectrum — and each interacts with matter in its own characteristic way.
One family of radiation, sorted from long radio waves to short gamma rays.
Wavelength and frequency move in opposite directions: as one goes up the other goes down, since their product is the fixed speed of light. Spectra are sometimes plotted against wavelength, sometimes frequency, and in infrared work often against wavenumber (the reciprocal of wavelength).