The electromagnetic spectrum is the entire range and scope (spectrum) of frequencies of electromagnetic radiation and their respective wavelengths and photon energies. The electromagnetic spectrum extends from below the low frequencies used for modern radio communication to gamma radiation at the short-wavelength (high-frequency) end, thereby covering wavelengths from thousands of kilometers down to a fraction of the size of an atom. Visible light lies toward the shorter end, with wavelengths from 400 to 700 nanometres. The limit for long wavelengths is the size of the universe itself, while it is thought that the short wavelength limit is in the vicinity of the Planck length.[4] Until the middle of the 20th century it was believed by most physicists that this spectrum was infinite and continuous. Nearly all types of electromagnetic radiation can be used for spectroscopy, to study and characterize matter.[5] Other technological uses are described under electromagnetic radiation.
- near-IR, approximately 14000–4000 cm−1 (0.8–2.5 μm wavelength)
- mid-infrared, approximately 4000–400 cm−1 (2.5–25 μm)
- far-infrared, approximately 400–10 cm−1 (25–1000 μm)