Electromagnetic spectrum
The range of electromagnetic radiation by frequency or wavelength, from radio waves to gamma rays. The optical part runs from the ultraviolet near 10 nm through the visible, 380 to 780 nm, to the far infrared at 1 mm; photon energies there span 124 eV down to about 1 meV.
Light is one part of a continuous range of electromagnetic waves that differ only in frequency. In order of increasing frequency, and decreasing wavelength, the named regions are radio, microwave, terahertz, infrared, visible, ultraviolet, X-rays and gamma rays. The boundaries are conventions based on how the radiation is produced and detected, and they overlap.
Wavelength ranges
| Region | Wavelength | Frequency |
|---|---|---|
| Radio | above 1 m | below 300 MHz |
| Microwave | 1 mm – 1 m | 300 MHz – 300 GHz |
| Terahertz | 30 µm – 1 mm | 0.3 – 10 THz |
| Infrared | 780 nm – 1 mm | 0.3 – 384 THz |
| Visible | 380 – 780 nm | 384 – 789 THz |
| Ultraviolet | 10 – 400 nm | 750 THz – 30 PHz |
| X-rays | about 0.01 – 10 nm | 30 PHz – 30 EHz |
| Gamma rays | below about 0.01 nm | above 30 EHz |
Wavelength and frequency are related by , so 1 m corresponds to 300 MHz and 500 nm to 600 THz. The terahertz band overlaps the far infrared, and X-rays and gamma rays are distinguished more by origin (electronic transitions against nuclear decay) than by wavelength; the 0.01 nm boundary, 124 keV, is only a rough guide.
The optical region
| Region | Wavelength | Photon energy |
|---|---|---|
| Extreme and vacuum ultraviolet | 10–200 nm | 124–6.2 eV |
| Ultraviolet (UV-C, UV-B, UV-A) | 100–400 nm | 12.4–3.1 eV |
| Visible | 380–780 nm | 3.26–1.59 eV |
| Near and short-wave infrared | 0.78–3 µm | 1.59–0.41 eV |
| Mid infrared | 3–50 µm | 0.41–0.025 eV |
| Far infrared | 50 µm–1 mm | 25–1.2 meV |
The visible limits of 380 and 780 nm are the CIE's; the eye's sensitivity is very low at both ends. The ultraviolet and infrared entries give their sub-bands, and the telecom bands are a small slice of the short-wave infrared from 1260 to 1675 nm. Converting between the columns uses ; see wavelength, frequency and wavenumber.
What changes across it
The photon energy decides how light interacts with matter. Ultraviolet photons, above a few electronvolts, break chemical bonds, ionize atoms in the extreme ultraviolet and damage biological tissue. Visible and near-infrared photons, around 1 to 3 eV, match the band gaps of semiconductors, which is why silicon detects visible light, why InGaAs is needed beyond 1.1 µm, and why telecom lasers are made from III-V alloys with gaps near 0.8 eV. Mid-infrared photons match molecular vibrations, so that region is used for spectroscopy and gas sensing, and the far infrared reaches rotational transitions and the thermal emission of cold objects. Transparency follows the same pattern: ordinary optical glass passes the visible and near infrared but starts to absorb below about 350 nm and beyond a few micrometres, which is why ultraviolet and mid-infrared optics use fused silica, calcium fluoride, sapphire, germanium or zinc selenide instead.
Measurement
Across the optical region, wavelength is measured with grating spectrometers and optical spectrum analyzers, and with Fourier-transform spectrometers in the infrared; the detector changes with the band, from photomultipliers and silicon in the ultraviolet and visible, to InGaAs, InSb and HgCdTe in the infrared, to bolometers in the far infrared.
The wavelength calculator converts a wavelength to frequency, photon energy and wavenumber and places it in the spectrum.
References: ISO 20473:2007, Optics and photonics: Spectral bands; ISO 21348:2007, Space environment: Process for determining solar irradiances; CIE S 017:2020, ILV: International Lighting Vocabulary.