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In wave propagation

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For periodic waves in nondispersive media (that is, media in which the wave speed is independent of frequency), frequency has an inverse relationship to the wavelength, λ (lambda). Even in dispersive media, the frequency f of a sinusoidal wave is equal to the phase velocity v of the wave divided by the wavelength λ of the wave: f = v λ . {\displaystyle f={\frac {v}{\lambda }}.} In the special case of electromagnetic waves moving through a vacuum, then v = c , where c is the speed of light in a vacuum, and this expression becomes: f = c λ . {\displaystyle f={\frac {c}{\lambda }}.} When waves from a monochrome source travel from one medium to another, their frequency remains the same—only their wavelength and speed change.
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Definitions

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For cyclical processes, such as rotation, oscillations, or waves, frequency is defined as a number of cycles per unit time. In physics and engineering disciplines, such as optics, acoustics, and radio, frequency is usually denoted by a Latin letter f or by the Greek letter ν {\displaystyle \nu } or ν (nu) (see e.g. Planck's formula). The relation between the frequency and the period, T {\displaystyle T} , of a repeating event or oscillation is given by f = 1 T . {\displaystyle f={\frac {1}{T}}.}
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Examples

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Light edit Visible light is an electromagnetic wave, consisting of oscillating electric and magnetic fields traveling through space. The frequency of the wave determines its color: 4 × 1014 Hz is red light, 8 × 1014 Hz is violet light, and between these (in the range 4- 8 × 1014 Hz ) are all the other colors of the visible spectrum. An electromagnetic wave can have a frequency less than 4 × 1014 Hz , but it 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 can have a frequency higher than 8 × 1014 Hz , but it will be invisible to the human eye; such waves are called ultraviolet (UV) radiation. Even higher-frequency waves are called X-rays, and higher still are gamma rays. All of these waves, from the lowest-frequency radio waves to the highest-frequency gamma rays, are fundamentally the same, and they are...
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