The distance between one peak or crest of a wave of light, heat, or other energy and the next corresponding peak or crest. In that light has qualities of waves, the various colors of the visible spectrum differ in the length of their waves, from violet at 400 nanometers to red at about 700 nanometers. Just shorter than violet's wavelength is that of ultraviolet light and then x-rays, both beyond the abilities of human eyes to see. Similarly, longer than red's, and invisible to the unassisted human eye is infrared. Wavelengths are usually measured in units of nanometers (nm) or of angstroms (?). 1 nanometer = 10 angstroms.Also see refraction, rhythm, and temperature.