Three typical vibrations (waves) are shown to the right.

The wavelength is the distance in centimeters, angstroms, or any other unit of length, from one wave crest to the next, and is represented by the Greek letter lambda (). Red light is electromagnetic radiation of wavelengths around 6800 , and progressing through the visible spectrum of red, orange, yellow, green, blue, and violet, the waves become shorter. Violet light has wavelengths around 4100 .

Radiation shorter than this cannot be perceived by the eye, and is in the ultraviolet region. Wavelengths longer than 7000 also are invisible and are termed infrared. The entire visible spectrum is shown in the margin of the preceding page, with the wavelengths that correspond to the various colors.