Editing Gaussian beam (section)

===Wavefront curvature===
The wavefronts have zero curvature (radius = ∞) at the waist. Wavefront curvature increases away from the waist, with the maximum rate of change occurring at the Rayleigh distance, {{math|1=''z'' = ±''z''<sub>R</sub>}}. Beyond the Rayleigh distance, {{math|{{mabs|''z''}} > ''z''<sub>R</sub>}}, the curvature again decreases in magnitude, approaching zero as {{math|''z'' → ±∞}}. The curvature is often expressed in terms of its reciprocal, {{mvar|R}}, the ''[[Radius of curvature (optics)|radius of curvature]]''; for a fundamental Gaussian beam the curvature at position {{mvar|z}} is given by:

<math display="block">\frac{1}{R(z)} = \frac{z} {z^2 + z_\mathrm{R}^2} ,</math>

so the radius of curvature {{math|''R''(''z'')}} is <ref name="svelto153" />
<math display="block">R(z) = z \left[{ 1+ {\left( \frac{z_\mathrm{R}}{z} \right)}^2 } \right].</math>
Being the reciprocal of the curvature, the radius of curvature reverses sign and is infinite at the beam waist where the curvature goes through zero.