The distance along the $z$ axis a beam travels from the _[beam waist](Gaussian%20beam%20radius.md#^a9abfb)_ of a [Gaussian beam](Gaussian%20beam.md) to a point at which the diameter has increased by a factor of $\sqrt{2}$ is referred to as the _Rayleigh range._ This is given as $z_R=\frac{\pi n w_0^2}{\lambda}=\frac{1}{2}b$ where $n$ is the [[Index of refraction]], $w_0$, is the beam radius at its waist, $\lambda$ is the wavelength, and $b$ is the [confocal parameter](Confocal%20parameter.md). The Rayleigh range, labeled as $z_R,$ is shown below in relation to the overall geometry of Gaussian beams.  <font size="2"> Here the light field distribution of a [Gaussian beam](Gaussian%20beam.md) is plotted on the $z$ axis and the Rayleigh range and [confocal parameter](Confocal%20parameter.md), $b$ are shown in relation to the beam geometry.</font> The Rayleigh range marks the approximate boundary between the [near-field propagation](Near-field%20propagation.md) length range and the [far-field propagation](Far-field%20propagation.md) length range in [Gaussian beam](Gaussian%20beam.md) optics. Following from this, the region between the beam waist and the Rayleigh range is often referred to as the _Fresnel region_, while the region outside is often referred to as the _Fraunhofer region._ %%Should the Fresnel region and Fraunhofer region warrant their own entries?%% #Electromagnetism/Optics/waveOptics