Answer: the electromagnetic spectrum is the complete range of electromagnetic radiation arranged in order of wavelength (or frequency), from gamma rays (shortest wavelength, highest energy) to radio waves (longest wavelength, lowest energy).
All electromagnetic radiations travel at the speed of light in vacuum, \(c = 3\times10^{8}\ \mathrm{m\,s^{-1}}\), and are related by
\[c = \nu\lambda,\qquad E = h\nu = \frac{hc}{\lambda}\]
so shorter wavelength means higher frequency and higher energy.
| Region | Approx. wavelength | Effect on molecules / use |
|---|---|---|
| Gamma rays | < 0.01 nm | Nuclear transitions |
| X-rays | 0.01–10 nm | Inner-shell electron transitions, crystal structure |
| Ultraviolet | 10–400 nm | Electronic transitions (UV spectroscopy) |
| Visible | 400–750 nm | Electronic transitions, colour |
| Infrared | 750 nm–1 mm | Molecular vibrations (IR spectroscopy) |
| Microwave | 1 mm–about 30 cm | Molecular rotations |
| Radio waves | longer than microwaves | Nuclear spin transitions (NMR) |
The boundaries are approximate and overlap. In spectroscopy, the region used decides what is studied: UV–visible for electronic energy levels, IR for bonds and functional groups, microwave for rotational levels and radio frequency for NMR.