The _Gouy phase_ or the _Gouy parameter_ of a [Gaussian beam](Gaussian%20beam.md) is given in terms of its [confocal parameter](Confocal%20parameter.md), $b$, or the [Rayleigh range](Rayleigh%20range.md), $z_R$ as $\eta(z)=\arctan{\bigg(\frac{2z}{b}\bigg)}=\arctan{\bigg(\frac{z}{z_R}\bigg)}.$ ^60cb66 As $z\rightarrow\pm\infty$ $\arctan{\big(\frac{2z}{b}\big)}\rightarrow\pm\frac{\pi}{2},$ following the form of an inverse tangent function. #Electromagnetism/Optics/waveOptics/GaussianBeamOptics