Monochromatic light traveling through a Michelson interferometer is split into two beams by a beam splitter. These beams travel to mirrors, reflect, and then recombine at the beam splitter. One of the mirrors can be moved. If the two mirrors are the same distance apart, they meet at the beam splitter in-phase and show constructive interference. As the one mirror is moved, the beams become slightly more out of phase. When the movable mirror has moved a distance equal to ¼ of the light's wavelength, the extra distance it travels is ½ the light's wavelength. This means that when the two beams combine at the beam splitter, they destructively interfere. So moving the mirror a total distance of ½ the wavelength of light means the spectra will show one complete cycle. One could then plot the power of radiation as a function of distance traveled by the movable mirror. However, by knowing the speed of the movable mirror the power of radiation can be plotted as a function of time.