= Beam divergence
{title2=$\theta$}
{wiki}
<Beam divergence> describes the angular spread of a beam away from its <optical axis>; specify the <radius> convention and whether the quoted <angle> is a half-angle or full <angle>. For the fundamental <Gaussian beam> in <vacuum>, the far-field $1/e^2$ <light intensity> <radius> obeys
$$
w(z)\sim\theta|z|,\qquad
\theta=\lim_{z\to\infty}\frac{w(z)}{z}=\frac{w_0}{z_R}=\frac{\lambda}{\pi w_0}.
$$
Here $\theta$ is the small-<angle> half-angle in radians under the <paraxial approximation>; the full <angle> is $2\theta$. The geometric asymptote has <angle> $\arctan(w_0/z_R)$, approximated by $w_0/z_R$ in this regime. Other beam profiles need their own <beam divergence> definitions and propagation laws.
Back to article page
