Editing Gaussian beam (section)

===Rayleigh range and confocal parameter===
{{main|Rayleigh length}}
The ''Rayleigh distance'' or ''Rayleigh range'' {{math|''z''<sub>R</sub>}} is determined given a Gaussian beam's waist size:

<math display="block">z_\mathrm{R} = \frac{\pi w_0^2 n}{\lambda}.</math>

Here {{mvar|λ}} is the wavelength of the light, {{mvar|n}} is the index of refraction. At a distance from the waist equal to the Rayleigh range {{math|''z''<sub>R</sub>}}, the width {{mvar|w}} of the beam is {{math|{{sqrt|2}}}} larger than it is at the focus where {{math|1=''w'' = ''w''<sub>0</sub>}}, the beam waist. That also implies that the on-axis ({{math|1=''r'' = 0}}) intensity there is one half of the peak intensity (at {{math|1=''z'' = 0}}). That point along the beam also happens to be where the wavefront curvature ({{math|1/''R''}}) is greatest.<ref name="svelto153" />

The distance between the two points {{math|1=''z'' = ±''z''<sub>R</sub>}} is called the ''confocal parameter'' or ''depth of focus'' of the beam.<ref>{{cite journal | last=Brorson | first=S.D. | date=1988 | title=What is the confocal parameter? | url=https://ieeexplore.ieee.org/document/155|journal=IEEE Journal of Quantum Electronics | volume=24 | issue=3 | pages=512–515 | doi=10.1109/3.155| bibcode=1988IJQE...24..512B | url-access=subscription }}</ref>