(color online) Three basic types of traveling waves. The left-side is plotted in the time domain; the right-side is plotted in the frequency domain. (a) A plane wave; its frequency ω is a constant. (b) A wave packet with narrow linewidth; is very small in comparison with its average frequency ω. The half-width of the wave packet in the time domain is . (c) A wave packet with large linewidth. Here also shown is the standard deviation of the Gaussian function.