(color online) A Gaussian distribution model of an electro–magnetic wave packet. (a) A 2D plot of the transverse component of the envelope function, . (b) The cross-section plot of along the x axis; σ is the standard deviation; a is the wave amplitude at the peak. (c) Variation of along the radius dimension. (d) A plot of the longitudinal component of the envelope function, , along the z axis. The wave packet moves at the speed c.