Color (continued)
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science
the invisible band of electromagnetic frequencies immedi-
ately adjacent to and less energetic than red light is known as
infrared radiation, while the invisible band more energetic than
violet light is called ultraviolet.
Receptors
The human eye is sensitive to the wavelength, or color, of light
because there are three different receptors—known as cones—
in the eye that respond to three different ranges of wave-
lengths, as illustrated in Fig. 3. The red (L) cones respond to
light of longer wavelengths (roughly 500–700 nm). The green
(M) cones respond to light of medium wavelengths (roughly
450–650 nm). The blue (S) cones respond to light of shorter
wavelengths (roughly 400–500 nm). The central wavelengths
for these three different ranges represent the three additive pri-
mary colors: red, green, and blue. The response of these three
receptors is used by the human brain to determine the color of
an object.
If light of a single well-defined wavelength enters the human
eye, then the degree to which it stimulates the red, green, or
blue receptors determines which color the observer perceives.
If only the red receptors (cones) are stimulated, then the brain
perceives red light. Similarly, if predominantly green recep-
tors are stimulated, then the brain perceives green. However, if
light at a single wavelength stimulates both the green and red
receptors, then the brain perceives yellow, which lies between
red and green in wavelength as shown in Fig. 3. While both re-
ceptors can be stimulated by light of a single wavelength, they
can also be stimulated in the same way with separate sources
of light at different wavelengths. Because a single wavelength
of yellow light and two wavelengths of green and red light can
produce the same visual stimulus within the brain, the brain
perceives the same yellow color in both cases. This is how
primary colors can be mixed to make any color in the rainbow
and is the basis of color displays in televisions, cell phones,
and computer screens, for instance. Within each pixel of the
display device, there are three different emitters for each of the
three primary colors (red, green, and blue). The brightness of
each emitter can be independently adjusted to stimulate an
arbitrary response in the eye of the viewer for each pixel—thus,
Fig. 2: Extensive chemical weathering can remove essential minerals and elements from soil. Typically, only the most insoluble compounds
remain in the soil. For example, iron oxides may persist, imparting a reddish-brown color to the soil. (Credit: Stephen Reynolds)
