white light consists of a mixture of various colors. The colors — identified as violet,
indigo, blue, green, yellow, orange, and red — represent the visible spectrum. The
human eye only sees “visible” light, which is any electromagnetic radiation falling
within the wavelength range of approximately 400 to 700 nm. Color perception is
related to wavelength; for example, radiation at 450 nm is observed as blue light, at
550 nm as green, and at 650 nm as red. When each color (wavelength) of the visible
spectrum is present with the same relative intensity, we have white light (Figure 3.8).
If each color (wavelength) in the visible region is not present at the same intensity
— such is the case when some colors are absent — then we have colored light.
(Black is the “color” perceived by the brain when no visible light is present, but this
does not preclude the presence of nonvisible radiation such as ultraviolet, infrared,
or x-rays.)