1.
“Light” is the narrow range of radiant energy that can stimulate
special receptors in the human eye which permit vision.
What
the human eye sees as light, such as sunlight or the glow from a candle,
is actually radiant energy. There is a broad spectrum of radiant energy, a
very small amount of which we can actually see.
When radiant energy within this range enters the eye, it stimulates
receptors along the inside back wall of the eyeball, which then transmit
information through the optic nerve to the brain. The eye-brain system
interprets the different energies of light received as different colors.
If radiant energy beyond the visible range enters the eye, it does not
stimulate the receptors in the eye and there is only darkness. No light,
no sight.
A
source that emits light that appears “white” is actually emitting
radiant energy over the entire range of visible light. If this white light
is passed through a prism, it can be separated into individual energies or
“colors” of light. This spread of light energies is commonly known as
the color spectrum. From highest to lowest energy, the color spectrum is:
VIOLET BLUE GREEN YELLOW ORANGE RED. (See appendix page A2 for
transparency master.)
The color spectrum can be thought of more simply as being composed of
three regions or colors: blue, green and red. These are known as the
primary colors of light.
When the three primary colors of light – blue,
green, red – are mixed together in equal amounts and enter our eyes, we
see white light. (See appendix page A3 for transparency master.)
2.
Mixing colors of light is a color addition process. When light of
different energies (or colors) enters the eye, the eye-brain system
perceives a color that is a combination of these.
When
two primary colors of light are mixed together in equal amounts, both
primary colors of light enter the eye, and the eye-brain system perceives
a color that is the combination of the two. Thus, mixing colors of light
is a color addition process. The color produced when two primary colors
are added together is called a secondary color of light. The secondary
colors of light are magenta (produced from red + blue), cyan (produced
from blue + green), and yellow (produced from red + green).
Any
secondary color of light, when added in the right amount to its opposite
primary color on the diagram above, will produce white light. For example,
when magenta light (red light and blue light) is added to green light, the
result is white light. Magenta and green are known as complementary
colors, as are cyan and red, and yellow and blue.
3.
Color is not a property that is a physical part of the things we see, but
the effects of light bouncing off or passing through those physical
things. Only the light that enters the eye from an object determines what
“color” is seen.
Light
is necessary to see the world around us. In order to see an object, light
must shine on it, and then be reflected or transmitted from the object to
our eyes. (The light shining on the object may come directly from a light
source, or may be reflected from other surfaces onto the object.) It is
the light reflected or transmitted from it to our eyes that determines the
color we perceive it to be.
A
white object reflects almost all of the light falling on it.
A black object absorbs almost all of the light falling on it and reflects
very little to our eyes. Consequently we see the absence of color, or
black. (The object or surface must reflect SOME light or we would not be
able to see it at all.) Most colored objects contain pigments that cause
matter to appear colored by absorbing certain colors of light and
reflecting the rest. We see the colors that are reflected back to our
eyes. For example, when white light shines on a green leaf, the
chlorophyll pigment in the leaf absorbs most of the colors that make up
white light and reflects only the green.
In
reality, most pigments are not pure. They reflect not just one color, but
many closely related colors. The green leaf, in addition to green light,
probably reflects some blue and yellow light.
R
O Y G B
V Y G B
Green
Pigment
The
relative amounts of green, blue and yellow light reflected determine the
exact shade of green. A blue pigment, such as the dye used in blue
jeans, absorbs most colors of light and reflects blue, and probably some
green and violet.
R
O Y G B
V
G B V
Blue
Pigment
A
primary color of pigment is defined as one that
absorbs (or subtracts) a primary color of light and reflects the other
two. The primary colors of pigment are magenta, cyan and yellow -- the
same as the secondary colors of light!
A magenta pigment absorbs green
light and reflects red and blue light to our eyes. Hence we see a
reddish-blue color, or magenta.
WHITE
MAGENTA
R
G
B
R B
Magenta
pigment
A cyan pigment absorbs red light and
reflects blue and green to our eyes. Hence we see a bluish-green color, or
cyan.
WHITE
CYAN
R G
B
G B
A yellow pigment absorbs blue light, and reflects red and
green to our eyes. Hence we perceive the combination of red and green
light, or yellow.
WHITE
YELLOW
R
G
B
R G
NOTE:
Most elementary students learn that the primary pigments are red, blue and
yellow rather than magenta, cyan and yellow. Red pigment is very similar
to magenta pigment (although magenta pigment reflects more blue light),
and blue pigment is very similar to cyan pigment (although cyan pigment
reflects more green light). Because red, blue, and yellow are approximate
versions of primary pigments, and are more familiar colors to youngsters,
they are sometimes called the primary pigments. They are also the versions
of primary pigments used by artists. Such approximations work fine with
young children and artists, but cause problems for older students who are
studying the behavior of light and color.
A
filter is a material or device that lets certain things pass through but
blocks other things. A color filter allows only certain colors of light to
pass through it by absorbing all of the rest of the colors. Examples of
color filters include colored cellophane, water to which a drop of food
coloring has been added, green bottle glass, etc. It is the color(s) of
light passing through a filter to our eyes that determines what color the
filter appears to be. A pure filter lets only a single color of light pass
through it. In reality, most filters, like most pigments, are not pure and
let some other colors of light through. For example, bottle glass lets
some green light through and probably a little yellow light and blue
light.
The
color of objects also depends on the color(s) of light falling on them.
For example, consider an object that is red. Such an object absorbs all
colors except red, and reflects red to our eyes. If only blue light falls
on the object, it is absorbed. No light is reflected to our eyes and the
object appears black.
As
another example, consider a banana. A banana is yellow because it reflects
both red and green light to our eyes. (Remember that red and green are
primary colors of light that combine together to make yellow light.) If
only red light shines on the banana, the banana will look red. If only
green light shines on the banana, it will look green. But if only blue
light shines on the banana, it will look black.
WHITE
YELLOW
RED
RED
BLUE BLACK
RGB
R
G
R
R
B
This latter example
clearly illustrates that color is not an inherent property of an object,
but a property of the light that the object reflects (or transmits).
4. Mixing colors of pigments or filters is a color subtraction process.
Each pigment or filter removes certain colors of light from the visible
spectrum, so that only light that is reflected by all pigments or that
passes through all filters enters our eyes and determines the color we
see.
When
pigments are mixed, each pigment removes (or subtracts) certain colors of
light from the visible spectrum. Only the light reflected by all of the
pigments enters the eye. For example, when cyan paint and yellow paint are
mixed, the result is green paint.
Mixing colored filters is also a
color subtraction process. When colored filters are mixed, or actually
layered on top of one another, each filter absorbs (or subtracts) certain
colors from the light that passes through it. Only light that can pass
through all of the filters is transmitted to the eye. For example, when
white light passes through a cyan filter and then a yellow filter, the
emerging light is green.
White
Green
cyan filter yellow
filter
5.
A trichromatic model can be used to explain color vision. The normal human
eye contains three kinds of color receptor cells, each sensitive to light
from one of three overlapping regions of the color spectrum – red,
green, and blue.
Just
as only three colors of light- red, blue, and green- are needed to produce
all colors of the visible spectrum, so does your eye only need three kinds
of receptors - red, blue, and green - in order for you to see all colors.
This is illustrated in the diagram below, which your workshop leader will
further explain.
The retina contains specialized cells that are sensitive
to light. When light from some scene enters the eye and is imaged in the
retina, the light sensitive cells absorb some of the light. The absorption
of light by these cells initiates a series of electrical impulse signals
that move from the light-sensitive cells along a pathway to the visual
cortex region of the brain. The processing of the signals in the brain
results in our perception of the light. There are two types of
light-sensitive cells on the retina. They are the rods and the cones. The
rods are only functional under dim light conditions like, for example, the
conditions outside at night. We receive no information about the colors of
objects from the rods. The cones are most functional under bright light
conditions; they do not function very well under dim light condition. When
cones absorb light, the information sent to the brain concerns information
that enables us to perceive both the intensity and the color of the light.
There are three different types of cone cells. Each type of cone cell
absorbs light from a different part of the spectrum. For example, one type
of cone cells will absorb a large fraction of light in the violet and blue
part of the spectrum, but will absorb very little in the remaining part of
the spectrum. Therefore, when blue light enters the eye and strikes this
type of cone cell on the retina, there will be a large response— a large
signal transmitted to the brain. On the other hand, if red light strikes
this type of cone, there is almost no response. We will call this type of
cone cell a “blue-sensitive” cone. The graph shown on the previous
page is a sketch of a curve showing the relative response of the “blue-sensitive”
cone as a function of the spectral color of the light striking it. In
addition to the “ blue-sensitive” cones, there are two other types of
cones, which we call the “green-sensitive” cones and the “red-sensitive”
cones. Their response as a function of spectral color are also shown in
the previous sketch. Note that what we refer to as the “green-sensitive”
cones actually respond to a wide range of spectral colors, not just green.
We use the terms “green-sensitive” and “red-sensitive” mainly for
convenience.
