The electromagnetic (EM) spectrum is the range of all possible electromagnetic radiation frequencies. The "electromagnetic spectrum" (usually just spectrum) of an object is the characteristic distribution of electromagnetic radiation from that particular object.

The electromagnetic spectrum extends from below the frequencies used for modern radio (at the long-wavelength end) through gamma radiation (at the short-wavelength end), covering wavelengths from thousands of kilometres down to a fraction the size of an atom. It is thought that the short wavelength limit is in the vicinity of the Planck length, and the long wavelength limit is the size of the universe itself (see physical cosmology), although in principle the spectrum is infinite and continuous.

Range of the spectrum

Electromagnetic energy at a particular wavelength λ (in vacuum) has an associated frequency f and photon energy E. Thus, the electromagnetic spectrum may be expressed equally well in terms of any of these three quantities. They are related by the equations:

$c\; =\; \backslash ,\backslash !$frequency x wavelength or $\backslash lambda\; =\; \backslash frac\{c\}\{f\}\; \backslash ,\backslash !$ and $E=hf\; \backslash ,\backslash !$ or $E=\backslash frac\{hc\}\{\backslash lambda\}\; \backslash ,\backslash !$

Where $c=299,792,458\backslash ,\backslash !$m/s (speed of light) and $h\backslash ,\backslash !$ is Planck's constant, $(h\; \backslash approx\; 6.626069\; \backslash cdot\; 10^\{-34\}\; \backslash \; \backslash mbox\{J\}\; \backslash cdot\; \backslash mbox\{s\}\; \backslash approx\; 4.13567\; \backslash \; \backslash mathrm\{\backslash mu\}\; \backslash mbox\{eV\}/\backslash mbox\{GHz\})$.

So, high-frequency electromagnetic waves have a short wavelength and high energy; low-frequency waves have a long wavelength and low energy.

Whenever light waves (and other electromagnetic waves) exist in a medium (matter), their wavelength is decreased. Wavelengths of electromagnetic radiation, no matter what medium they are traveling through, are usually quoted in terms of the vacuum wavelength , although this is not always explicitly stated.

Generally, EM radiation is classified by coiled wavelength into radio wave, microwave, infrared, the visible region we perceive as light, ultraviolet, X-rays and gamma rays.

The behavior of EM radiation depends on its wavelength. Higher frequencies have shorter wavelengths, and lower frequencies have longer wavelengths. When EM radiation interacts with single atoms and molecules, its behavior also depends on the amount of energy per quantum it carries. Electromagnetic radiation can be divided into octaves — as sound waves are.

Spectroscopy can detect a much wider region of the EM spectrum than the visible range of 400 nm to 700 nm. A common laboratory spectroscope can detect wavelengths from 2 nm to 2500 nm. Detailed information about the physical properties of objects, gases, or even stars can be obtained from this type of device. It is widely used in astrophysics. For example, many hydrogen atoms emit a radio wave photon which has a wavelength of 21.12 cm.