Visible Light and the Eye's Response
As mentioned in the first section of Lesson 2, our eyes are sensitive to a very narrow band of frequencies within the
enormous range of frequencies of the electromagnetic spectrum. This narrow band
of frequencies is referred to as the visible light spectrum. Visible light -
that which is detectable by the human eye - consists of wavelengths ranging
from approximately 780 nanometer (7.80 x 10-7 m) down to
390 nanometer (3.90 x 10-7 m).
Specific wavelengths within the spectrum correspond to a specific color based upon how humans typically perceive light
of that wavelength. The long wavelength end of the spectrum corresponds to
light that is perceived by humans to be red and the short wavelength end of the
spectrum corresponds to light that is perceived to be violet. Other colors within the spectrum include orange, yellow,
green and blue. The graphic below depicts the approximate range of wavelengths
that are associated with the various perceived colors within
the spectrum.
Color Cones
Color can be
thought of as a psychological and physiological response to light waves of a
specific frequency or set of frequencies impinging upon the eye. An
understanding of the human response to color demands
that one understand the biology of the eye. Light that enters the eye through
the pupil ultimately strikes the inside surface of the eye known as the retina. The retina is lined with a variety of light sensing cells known as
rods and cones. While the rods on the retina are sensitive to the intensity of
light, they cannot distinguish between lights of different wavelengths. On the
other hand, the cones are the color-sensing
cells of the retina. When light of a given wavelength enters the eye and
strikes the cones of the retina, a chemical reaction is activated that results
in an electrical impulse being sent along nerves to the brain. It is believed
that there are three kinds of cones, each sensitive to its own range of
wavelengths within the visible light spectrum. These three kinds of cones are
referred to as red cones, green cones, and blue cones because of their
respective sensitivity to the wavelengths of light that are associated with
red, green and blue. Since the red cone is sensitive to a range of wavelengths,
it is not only activated by wavelengths of red light, but also (to a lesser
extent) by wavelengths of orange light, yellow light and even green light. In
the same manner, the green cone is most sensitive to wavelengths of light
associated with the color green. Yet the
green cone can also be activated by wavelengths of light associated with
the colors yellow and blue. The graphic
below is a sensitivity curve that depicts the range of wavelengths and the
sensitivity level for the three kinds of cones.
The cone sensitivity curve shown
above helps us to better understand our response to the light that is incident
upon the retina. While the response is activated by the physics of light waves,
the response itself is both physiological and psychological. Suppose that white
light - i.e., light consisting of the full range of wavelengths within the
visible light spectrum - is incident upon the retina. Upon striking the
retina, the
physiological occurs: photochemical reactions occur within the cones to produce
electrical impulses that are sent along nerves to the brain. The cones respond
to the incident light by sending a message forward to brain, saying,
"Light is hitting me." Upon reaching the brain, the psychological
occurs: the brain detects the electrical messages being sent by the cones and
interprets the meaning of the messages. The brain responds by saying "it
is white." For the case of white light entering the eye and striking the
retina, each of the three kinds of cones would be activated into sending the
electrical messages along to the brain. And the brain recognizes that the
messages are being sent by all three cones and somehow interprets this to mean
that white light has entered the eye.
Now suppose that light in the yellow range of wavelengths
(approximately 577 nm to 597 nm) enters the eye and strikes the retina. Light
with these wavelengths would activate both the green and the red cones of the
retina. Upon striking the retina, the physiological occurs: electrical messages
are sent by both the red and the green cones to the brain. Once received by the
brain, the psychological occurs: the brain recognizes that the light has
activated both the red and the green cones and somehow interprets this to mean
that the object is yellow. In this sense, the yellow appearance of objects is
simply the result of yellow light from the object entering our eye and
stimulating the red and the green cones simultaneously.
If the appearance of yellow is perceived of an object when it
activates the red and the green cones simultaneously, then what appearance
would result if two overlapping red and green spotlights entered our eye? Using
the same three-cone theory, we could make some
predictions of the result. Red light entering our eye would mostly activate the
red color cone; and green light entering
our eye would mostly activate the green color cone.
Each cone would send their usual electrical messages to the brain. If the brain
has been psychologically trained to interpret these two signals to mean
"yellow", then the brain would perceive the overlapping red and green
spotlights to appear as yellow. To the eye-brain system, there is no difference
in the physiological and psychological response to yellow light and a mixing of
red and green light. The brain has no means of distinguishing between the two
physical situations.
In a technical sense, it is really not appropriate to refer
to light as being colored. Light is simply a
wave with a specific wavelength or a mixture of wavelengths; it has no color in and of itself. An object that is emitting or
reflecting light to our eye appears to have a specific color as
the result of the eye-brain response to the
wavelength. So
technically, there is really no such thing as yellow light. Rather, there is
light with a wavelength of about 590 nm that appears yellow. And there is also
light with a mixture of wavelengths of about 700 nm and 530 nm that together
appears yellow. The yellow appearance of these two clearly different light
sources can be traced to the physiological and psychological response of the
eye-brain system, and not to the light itself. So to be technically
appropriate, a person would refer to "yellow light" as "light
that creates a yellow appearance." Yet, to maintain a larger collection of
friendships, a person would refer to "yellow light" as "yellow
light."
In the next several sections of Lesson 2, we will explore
these concepts further by introducing three primary colors of
light and generating some simple rules for predicting the color appearance of objects in terms of the three
primary colors.