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.