Moving waves can be described by their frequency, or number of waves per second, as well as by their wavelength. If a train of waves is moving past a stationary observer at a frequency of (nu) complete cycles, or waves per second, and if each of these has a wavelength of (lambda) centimeters per cycle, then the wave must be moving at a speed of = v cm per second.

This is illustrated with boxcars and with orange light on this page and the next.