Frequency

=== Light ===
{{main article|Light|Electromagnetic radiation}}
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[[File:EM spectrum.svg|thumb|upright=2|Complete spectrum of [[electromagnetic radiation]] with the visible portion highlighted]]

Visible light is an [[electromagnetic wave]], consisting of oscillating [[electric field|electric]] and [[magnetic field]]s traveling through space. The frequency of the wave determines its color: 400 THz ({{val|4|e=14|ul=}} Hz) is red light, 800 THz ({{val|8|e=14|u=Hz}}) is violet light, and between these (in the range 400–800 THz) are all the other colors of the [[visible spectrum]]. An electromagnetic wave with a frequency less than {{val|4|e=14|u=Hz}} will be invisible to the human eye; such waves are called [[infrared]] (IR) radiation. At even lower frequency, the wave is called a [[microwave]], and at still lower frequencies it is called a [[radio wave]]. Likewise, an electromagnetic wave with a frequency higher than {{val|8|e=14|u=Hz}} will also be invisible to the human eye; such waves are called [[ultraviolet]] (UV) radiation. Even higher-frequency waves are called [[X-ray]]s, and higher still are [[gamma ray]]s.

All of these waves, from the lowest-frequency radio waves to the highest-frequency gamma rays, are fundamentally the same, and they are all called [[electromagnetic radiation]]. They all travel through vacuum at the same speed (the speed of light), giving them wavelengths inversely proportional to their frequencies.
<math display=block qid=Q2111>\displaystyle c=f\lambda,</math>
where ''c'' is the speed of light (''c'' in vacuum or less in other media), ''f'' is the frequency and ''λ'' is the wavelength.

In [[Dispersion (optics)|dispersive media]], such as glass, the speed depends somewhat on frequency, so the wavelength is not quite inversely proportional to frequency.