linguistic:voice_change, structural:summarization
Beam divergence

Although the tails of a Gaussian function never actually reach zero, for the purposes of the following discussion the "edge" of a beam is considered to be the radius where r = w (z). That is where the intensity has dropped to 1/ e of its on-axis value. Now, for z ≫ z the parameter w (z) increases linearly with z. This means that far from the waist, the beam "edge" (in the above sense) is cone-shaped. The angle between that cone (whose r = w (z)) and the beam axis (r = 0) defines the divergence of the beam: θ = lim z → ∞ arctan (w (z) z). {\displaystyle \theta =\lim _{z\to \infty }\arctan \left({\frac {w(z)}{z}}\right).}