= Rayleigh length
{c}
{title2=$z_R=\pi w_0^2/\lambda$}
{wiki}
= Rayleigh range
{c}
{synonym}
The <Rayleigh range> of a fundamental <Gaussian beam> in <vacuum> is the positive axial distance from the <beam waist> at which its <radius> grows from $w_0$ to $\sqrt2w_0$. Thus its contour <area> doubles and its peak <irradiance> halves. Within $|z|\leq z_R$, the <radius> stays at most $\sqrt2w_0$; for $|z|\gg z_R$, <beam divergence> dominates. At fixed <vacuum> <wavelength>, the <Rayleigh range> scales quadratically with the <beam waist> <radius>. The formula here uses the <vacuum> <wavelength>, not an unlabelled <wavelength> in a material.
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