INVESTIGATION
Collaborative Learning
B2
This investigation is Exploros-enabled for tablets. See page xiii for details.
B2 Additive Color Model and Vision
Key Question: How do we see color?
In this investigation, students explore what happens
when they mix different colors of light. They use
flashlights with color filters to project colors of light onto
a screen. Then, they observe the result when they mix
different combinations of the three primary colors of
light: red, green, and blue. They apply what they learned
to human vision and how we perceive color. Finally,
they use diffraction glasses to observe light from the
flashlights. This allows them to see that white light is
made of all of the colors of light.
Learning Goals
✔✔Use flashlights to mix primary colors of light and show
that white light can be made from red, green, and
blue light.
✔✔Compare sources of light.
✔✔Explain how humans see color.
GETTING STARTED
Time 50 minutes
Setup and Materials
1. Make copies of investigation sheets for students.
2.
W
atch the equipment video.
3. Review all safety procedures with students.
Materials for each group
yy Optics with Light & Color kit
O
nline Resources
Available at curiosityplace.com
yy E quipment Video: Optics with Light & Color
yy Skill and Practice Sheets
yy Whiteboard Resources
yy Animation: Additive Color Model
yy Science Content Video: RGB Color Model
yy Student Reading: Vision and Color
Vocabulary
additive color model – a process that creates color by
adding proportions of red, green, and blue light together
color – a property of visible light that is related to its
wavelength
cone cells – photoreceptors on the surface of the retina
that respond to color
diffraction grating – an optical device consisting of an
assembly of parallel narrow slits or grooves that interfere
with incident radiation to disperse light waves, and can
result in spectra
photoreceptors – light-sensitive cells on the surface of
the retina of the eye
pixel – the smallest element in a display or image
rod cells – photoreceptor cells in the retina of the eye
that respond to differences in brightness
visible light – the light you can see in the range between
400 and 700 nanometers
white light – light containing an equal mix of all colors
NGSS Connection This investigation builds conceptual understanding and skills for the following performance expectation.
HS-PS4-3. Evaluate the claims, evidence, and reasoning behind the idea that electromagnetic radiation can be described
either by a wave model or a particle model, and that for some situations one model is more useful than the other.
Science and Engineering Practices
Engaging in Argument from Evidence
Disciplinary Core Ideas
PS4.A: Wave Properties
PS4.B: Electromagnetic Radiation
Crosscutting Concepts
Systems and System Models
Optics with Light and Color
51
Additive Color Model and Vision
BACKGROUND
Every time humans see something, light is involved. Like
heat and sound, light is a form of energy. Light comes
to our eyes in two ways: directly from a light-producing
object, like a star, or reflected from objects that do not
produce their own light, like the paper page of a book.
When we see things in good light, we usually perceive a
color. The sky tends to be blue in the middle of the day,
or it might have reds and oranges in the evening. A fire
alarm is usually red; things like walls and shirts may be
any color including black and white.
White light is a combination of all the colors of visible
light. A dispersive element like a prism or a diffraction
grating interferes with light in a way that can separate
out the colors to show that white light is in fact made
up of many colors. Simply speaking, these dispersive
elements use refraction or diffraction to bend or change
the path of light. Light and other waves of different
wavelengths respond differently to these conditions,
essentially causing them to be separated out. If you have
seen light spectra from prisms or spectrometers, you are
seeing refracted or diffracted light, respectively.
Since light is energy, different colors are simply lights
of different energies. Color is how humans perceive
the energy of light. When we talk about colors of light,
we talk about wavelength. The double-slit experiment
conducted by Thomas Young (1773-1829) at the
beginning of the 19th century is the earliest published
experiment using diffraction gratings to show that light
must be made up of waves, and further, that different
colors have different wavelengths and frequencies.
All the colors of visible light can be created using
combinations of three primary colors: red, blue, and
green. While we could use other colors of light, we
generally use red, blue and green because we have
photoreceptors in the eye called cone cells that are
“tuned” to these three colors. We use the letters from the
colors, RGB, to describe this model of color creation and
perception. While most of us think that color is inherent
in all objects, an object’s color is really just a way we
perceive light of different energies or wavelengths that
are reflected or transmitted to our eyes.
52 The human eye also contains another type of lightsensing cell called a rod cell. Rod cells sense the overall
intensity of light and therefore see in black and white.
They do not see color. Because they can respond to all
colors, rod cells are much more sensitive, and can detect
lower levels of light, than cone cells. This is why colors
seem washed out in the dark. The lower the overall level
of light, the more the eye sees only black and white
images. There are about 130 million rod cells and only
about 7 million cone cells. This means the sharpness in
our vision comes mostly from our perception of black
and white. The color associated with each triplet of
three cone cells is associated with the brightness seen
by 60–100 rod cells. Essentially, the image that you see is
assembled in your brain from 130 million black and white
dots and 7 million colored dots.
In this investigation, students discover what happens
when different colors of light are mixed. We focus on the
additive color model because we are generally talking
about emitted light and transmitted light. By combining
colors of light, we are adding light energies together and
therefore sending more light to our eyes in the areas
that perceive those colors. We use the color filters in this
investigation create the three primary colors of light: red,
green and blue. We can then combine light from two or
more flashlights to demonstrate the additive model.
Televisions and computer monitors make red, green,
and blue (RGB) light directly (graphic below). Use a
magnifying glass to look closely at a white area on a TV
screen or computer monitor and you will see that what
appears white is actually tiny red, green, and blue dots.
The dots are called pixels and each pixel gives off its
own light. The pixels are separated by very thin black
lines. The black lines help give intensity to the colors and
help make the dark colors appear darker. From far away,
you cannot see the individual pixels. Instead, you see a
nice, smooth color picture. By turning on the different
color pixels at different
intensities, TV sets can mix
the three primary colors
to get millions of different
colors. For example, a
light brown color could be
displayed by illuminating
88 percent of the red, 85
percent of the green, and 70
percent of the blue pixels.
B2
5E LESSON PLAN
Engage
Elaborate
Turn out the lights and turn on a flashlight with no color
filter. Ask the students what they see. [White light.] If they
don’t describe the light, ask them to. Next, have them all
put on diffraction grating glasses and look at the light
again. Ask them once again to describe what they see. Ask
them what they think happened to the light.
Making Colored Shadows Using Additive Color Mixing
When they put on the glasses, they should see an array of
dashes of color in red, orange, yellow, green, and blue. This
demonstration will prepare them for understanding the
idea that the rainbow of colors came from the white light.
When an object is placed in the path of light, it is made
visible if you are able to view it from the side of the light.
But light also casts a shadow directly behind the object.
In making shadows, the object in the path of light is
called the occluding object. When there is more than one
source of light, multiple shadows can form. The regions
where some light gets by blocked are called penumbra.
The region where no light shines is called the umbra.
Demonstrate this using the following procedure:
Light A
Explore
Have students complete Investigation B2, Additive Color
Model and Vision. In this investigation, students view light
sources in three primary colors. They see how mixing
colors in different combinations creates new colors of
light. They discuss different sources of light that we use
every day, and learn how we see the light and images
all around us. Students will learn how we might see the
same color created by two different methods. Finally,
they see how filters work to create color.
Explain
Revisit the Key Question to give students an opportunity
to reflect on their learning experience and verbalize
understandings about the science concepts explored in
the investigation. Curiosityplace.com resources, including
student readings, videos, animations, and whiteboard
resources, as well as readings from your current
science textbook, are other tools to facilitate student
communication about new ideas.
Penumbra
B
Umbra
Penumbra
A
Light B
1. Connect the three flashlight holders in a line using the
slots and rails, arranging all three holders side by side.
2. Position the flashlights in the holders with the blue
light in the middle on the laminated grid. Align the
flashlights with their ends aligned with the edge of
the grid.
3. Fold a clean sheet of letter-sized paper in half and
tuck half under the back side of the closed box to
prop it up as a screen at the other end of the grid.
4. Turn off the classroom lights; the room should be as
dark as possible. Turn on the flashlights.
5. Place the laser in line with the center flashlight so that
it stands up like a pole about 10 cm (20 squares on
the grid) from the front of the flashlights, as shown in
the diagram below.
(continued on next page)
Science Content Video
RGB Color Model
Animation
Additive Color Model
Optics with Light and Color
53
Additive Color Model and Vision
Ask students to draw and describe the projected image.
[Students should see that there are three vertical bands
of color. Outside the bands is a whitish light. There are
two small overlapping sections at the base of the bands.]
Ask students, “How many different colors of light do
you see? What are they and where are they?” Then have
students make guesses as to how the colors were made.
cyan
yellow
magenta
Ask students how they can prove your explanation by
turning on only one light at a time. They might surmise
that if you turn off all but one light at a time, there is a
single shadow formed for each light. For each flashlight,
the position of the shadow is the location of the band
of color formed by the other two color lights. So, for
example, the blue light in the middle forms a shadow
in the middle. That shadow is where green and red
combine to form yellow.
Finally, ask students, with all the lights on, what are the
areas of color called, umbra or penumbra? The answer is
penumbra. Umbra is where no light shines. There are no
places where there is umbra from the pole.
whitish
whitish
Evaluate
yy D
uring the investigation, use the checkpoint
questions as opportunities for ongoing assessment.
green
red
The three colors combine to make the whitish light
that surrounds the bands. The band shapes match the
shape of the laser-pole. Since there are three distinct
light sources, the pole casts one shadow from each light
source, but light from the other lights shines there and
creates a penumbra. For each large band, the pole blocks
light from one light source, and the two unblocked colors
are mixing to create a new color: cyan from blue and
green, yellow from green and red, and magenta from
red and blue. In the smaller regions where we see green
and red, there is a “double” overlap of shadows. Two
light sources are blocked. There, you have one pure color
from each of the two outside light sources. In one of
the smaller sections, blue and red are blocked, showing
green. In the other small section, blue and green are
blocked, showing red.
54 yy A
fter completing the investigation, have students
answer the assessment questions on the Evaluate
student sheet to check understanding of the
concepts presented.
B2
Explore
INVESTIGATION
B2
Name ____________________________________________ Date ________________________
B2 Additive Color Model and Vision
How do we see color?
INVESTIGATION
B2
2. Put a different color filter on each of the three flashlights. Slide all three flashlights into the three holders
so that blue is on the top and green and red below.
Materials:
Optics with Light & Color kit:
• 3 Flashlights with holders
All the colors of visible light can be created artificially using a
combination of three primary colors: red, blue, and green. Don’t believe
it? You will use a white light source and color filters to discover what
happens when you mix different colors of light. You will also learn how
those filters work with your eye to create the impression of different
colors in your brain.
Safety
Explore
Avoid shining a laser or flashlights directly into the eye.
• Red, green, and blue color filters
• Laser flashlight
• Convex lens with light blue
holder
• Laminated grid
• Diffraction grating glasses
3. Position the flashlights on one side of the laminated grid.
4. Set the white box that the Optics with Light & Color kit comes in at the other edge of the laminated grid.
Open the lid straight up to function as a screen for reflecting the lights.
5. Turn the flashlights on.
Mixing primary colors of light

First, let’s explore what happens when you mix the primary colors of light: red, green, and blue. Follow the
procedures below.
6. Position the light blue lens with the split side facing down between the flashlights so the lights shine
through it onto the box. Slide it toward or away from the box until you see three sharp spots of color
overlapping each other on the box.
1. Connect two of the flashlight holders using the rail and slot connectors on the side. Leave the third
holder unconnected. Place the third connector on top of the two connectors, making a small pyramid
stack.
slot rail
slide holders together
pyramid
stack
Turning off the lights will help you to better see the images on the screen.
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B2 Additive Color Model and Vision
Optics with Light and Color
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B2 Additive Color Model and Vision
Optics with Light and Color
Guiding the INVESTIGATION
 Mixing primary colors of light
This photograph shows the proper setup for Part 1 of the investigation.
Convex lens in
light blue holder
flashlights with
color filter caps
use lens to focus
light here
Laminated table
position holder with
split side down
Box
Optics with Light and Color
55
Additive Color Model and Vision
ADDRESSING MISCONCEPTIONS
In the investigation, students mix colors of light
and learn about the additive color model. This
process is different than mixing paints or pigments,
which employs the subtractive color model. When
students mix red
and green light to
get yellow, most
are surprised by this
result. The same is
true when the three
colors mix to achieve
white light. You can’t
achieve anything like
that with paints and
dyes. Additive color
is about adding light,
not pigment. Mixing
non-white dyes on
paper, for instance,
tends to darken
colors because more
colored pigment
absorbs more light.
TEACHING TIP
When setting up the color flashlight activity, it is a
good idea to use the laminated grid as a positioning
guide and the white box for a screen. Position the
flashlights, lens, and box as shown in Part 1 of the
investigation for an ideal projection and mixing
of colors. You can also use a folded piece of white
paper to make a screen.
56 Explore
INVESTIGATION
B2
Explaining what you see

Follow the steps and use Table 1 to record the answers to the following questions about your observations.
a. Turn on just the blue and red flashlights. What color do you see when you mix blue and red light?
b. Turn on just the green and blue flashlights. What color do you see when you mix green and blue light?
c. Turn on just the red and green flashlights. What color do you see when you mix red and green light?
d. Turn on all three flashlights. What color is produced when all three colors of light are equally mixed?
Table 1: Mixing primary colors of light
Color combination
Color you see
blue + red
pink (magenta)
green + blue
sky blue (cyan)
red + green
yellow
white
red + green + blue
Sources of light

a. Televisions, computer monitors, and cell phone display screens use dots of light to create images. These
dots are called pixels. What colors of light can these devices use to make all the possible colors needed
for the images they display?
All they need are red, green, and blue.
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B2 Additive Color Model and Vision
Optics with Light and Color
Guiding the INVESTIGATION
 Explaining what you see
Tell the students they are looking for where the
colors overlap each other. Let students share guesses.
After they mix the colors, use the questions in Part 2
to lead a discussion about students’ findings. Mixing
red and blue produces the color magenta. When
blue and green are mixed, the resulting color is light
blue (cyan). Mixing red and green creates yellow.
When all three colors of light are equally mixed, white
light is created. This will help students see clearly
that white light is a mixture of many colors. If the
students can get three circles of equal intensity to
overlap the screen, what they will see is a pretty clear
white spot in the area where the red, green, and blue
circles overlap.
B2
Guiding the INVESTIGATION
 Sources of light
Tell students, “Television screens use the three
primary colors of light to create the images humans
see. If you examine a television with a magnifying
glass, depending on the resolution of the screen,
you will see that the screen is made of tiny red,
green, and blue dots of light. By turning on the tiny
lights at different intensities and times, television
screens can mix these three colors to make millions
of different colors. From far away, your eyes cannot
see the tiny dots, but instead blend them together
to see a nice clear picture.” Demonstrate this with a
classroom television (or other electronic display) and
magnifying glasses if they are available.
Explore
INVESTIGATION
B2
b. How do you think they use these colors to create images?
They use pixels in the colors of red, green, and blue to create
regions of color that when seen together create an overall image.
c. Look at your clothes. Does the light reaching your eyes from your clothes originate in your clothes, or
does it come from somewhere else?
The light does not come from my clothes; it comes from lights and
the sun.
d. What color would your clothes appear to be in a room with no light?
The clothes can’t be seen in the dark. They are definitely not
producing their own light, so they would appear to have no color at
all, and all I would see is black.
Researching how we see colors

Prompt students to steer the discussion of red, blue,
and green “dots” toward pixels. Most students will
relate the word pixel to the resolution of their digital
cameras or their mobile phone cameras. Allow a
few student volunteers to discuss how increasing
the resolution of the camera produces a better
quality image.
You can also start a discussion about why objects
appear a certain color. A shirt looks blue because
it reflects blue light, which means there must be
blue light in whatever light falls on that shirt to be
reflected. Turn off the lights and shine a red light on
a blue shirt. Students should observe that the blue
shirt looks black.
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B2 Additive Color Model and Vision
Optics with Light and Color
The students can also demonstrate this concept
themselves. Turn off the classroom lights and
have them cup the laser in their hand, shielding
out light where the red laser label is. With the
green filter on a flashlight, shine the green light
on the label. The letters on the label will appear
green but the background of the label is black.
This is because there is almost no red in the
green light, and thus no red is reflected to the
eye.
Blue cloth looks blue because there are dyes
in the cloth that absorb all colors of light
except blue. When white light falls on the
fabric, everything except blue is absorbed. Since
blue light is what is reflected to your eye, the shirt
looks blue.
Optics with Light and Color
57
Additive Color Model and Vision
Explore
INVESTIGATION
B2
a. Research and explain the following terms from the diagram above: cone cells, rod cells, retina, and optic
nerve. Which cells are responsible for sensing color?
Cone cells – photoreceptor, or light sensing, cells in the retina of
the eye that respond to color
Rod cells - photoreceptor cells in the retina of the eye that
respond to differences in brightness
Retina - section of light-sensitive tissue at the back of the eye onto
which images are focused by the lens of the eye
Optic nerve - The nerve that serves to transmit information from
the photoreceptors in your eye to your brain
b. Research and explain how the eye sees green light in terms of the photoreceptors in the eye.
When green light is detected by green cone cells in a particular area
in the eye, signals are sent down the optic nerve to the brain that
green is being detected. The brain then sees green in that area.
c. Research and explain how the eye sees yellow light in terms of the photoreceptors in the eye.
There are two ways yellow can be seen. One way is for yellow light
to be detected by photoreceptors in the retina. Both red and green
cone cells are sensitive to yellow light. When both are stimulated in
a particular area, they both send a signal to the brain via the optic
nerve, and yellow is seen in that area.
Another way is for green and red light to enter the eye together in
a particular area, which results in both red and green cone cells
sending signals to the brain via the optic nerve, and the brain
interprets this as yellow in that area.
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B2 Additive Color Model and Vision
Optics with Light and Color
Guiding the INVESTIGATION
 Researching how we see colors
Tell students, “The experiments you completed today
are as much about human perception as they are about
physics. For example, when you mixed green light and
red light you saw yellow. You probably did not expect
to see yellow because yellow is a light color and red
and green seem like darker colors. You see yellow
because of how the human eye and brain detect and
perceive light. The retina of the eye has three types
of color sensors called cone cells. Each type of cone
cell is receptive to a particular energy of light. When
high energy light is received by the eye, only the blue
cone cells send signals to the brain. We have learned
to associate these signals with the name blue. Suppose
the cone cells for medium energy are active. What
58 Explore
INVESTIGATION
B2
d. Research and explain how the eye sees white light in terms of the photoreceptors in the eye.
White light stimulates the three kinds of cone cells. The red, green,
and blue cone cells send signals to the brain via the optic nerve
indicating all three colors are present in a particular area, and the
brain sees white in that area.
Examining color filters

A red laser can produce a single pure red light that can be useful in some applications. For example, making
three-dimensional images called holograms requires the very pure source of light that a laser can provide.
In the Optics with Light & Color kit, your flashlights with filters give you three different sources of colored
light, but just how “pure” are these colors? In this part of the investigation, you will examine the light produced
by each flashlight and learn how a color filter works.
1. Put on a pair of diffraction grating glasses.
2. Turn on a flashlight. Tilting it only slightly toward your eyes, observe the pattern you see.
3. Put each of the color filter caps on the flashlight and observe the colors you see for each.
a. What colors did you see with the white light? Which colors seemed strongest?
Violet, blue, green, yellow, orange, red. Blue, green, and red seem to
be the predominant colors.
b. Why do you think you saw all these colors?
The white light from the flashlight is made up of light of many
different colors.
c. What colors did you see with the colored caps?
Blue filter cap – predominantly blue, some red and a little green
Green filter cap - predominantly green, some blue and very little red
and yellow
Red filter cap - predominantly red, with yellow, green, and blue
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B2 Additive Color Model and Vision
Optics with Light and Color
color do you see?” Students should answer green. We
see green when the brain gets signals from cone cells
which respond to medium energy light.
Then ask, “What happens if the energy of light is
between low and medium? The low energy cone
cells and the medium energy cone cells both send
signals to the brain. What color do you see?” Students
learned from the
experiment that
we see a color
that we call yellow
when there is an
equally strong
signal from both
the red cone cells
and the green
cone cells.
B2
Guiding the INVESTIGATION
 Examining color filters
Explain to students that a film like the one in the
diffraction grating glasses is a transparent material
with a series of ridges or lines that cause light
to bend or diffract. Different colors of light have
different wavelengths. The effect of diffracted light
is different for different wavelengths, so you can see
the different wavelengths that make up the light.
Using a diffraction grating, it is easy to show that
white light is composed of many colors. Demonstrate
this point if time permits. Have students pick up the
diffraction grating glasses and look at a light source
(such as one of the flashlights) through them. They
should see a central bright spot and rainbows up,
down, and to the side. The glasses separate light into
a spectrum that shows the component colors in the
rainbows on the sides of the central bright spot.
Explore
B2
d. Did you see the color red when you viewed the different filtered light through the diffraction grating
glasses? Rank the filters according the amount of red you saw with each, with 1 being the most red
and 3 being the least.
1-red filter, 2-blue filter, 3-green filter
4. Turn on the laser, shining it at an angle toward the ceiling. Observe the light with the diffraction grating
glasses.
a. What colors do you see?
Only red.
5. Now put the red filter on the laser, aiming the light at the palm of your other hand a few inches away.
Repeat this with the blue and green filters.
a. Describe what happened with each filter.
The red filter allowed the red laser light to reach my palm clearly.
The blue filter allowed some of the red laser light to reach my palm,
but faintly. The green filter allowed no light to reach my palm.
b. Based upon your experience with the laser and the filters, explain why we call them filters.
The filters allow only some light through them. Some light is
absorbed or blocked.
c. Based upon your experience with the white light and the diffraction grating, explain what you saw
with the laser and the filters.
The white light is made up of all colors, which you can see when we
disperse the light with the diffraction grating glasses. When you
apply the filters, only certain colors are allowed through. Red filters
let the most red through and green filters let the least.
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Can be duplicated for classroom use
If students use the diffraction grating glasses to look
at each of the lights with color filters, they should
observe that each of the colored lamps has a spectrum.
A light looks green if green is the dominant color, even
if the light includes other colors as well, as with the
green light students produce in this investigation. For
example, the red lamp has mostly red, but it also has
some green and a blue. The blue lamp has mostly blue
but it also has a small amount of red and even some
green. The green has a little blue but almost no red.
Have students compare the spectrum they see from
the red, green, and blue lights with each other. They
will rank the filtered light by which one has the most
red and which one has the least.
INVESTIGATION
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B2 Additive Color Model and Vision
Optics with Light and Color
The color filter works by removing colors from light.
If a light source has only one wavelength, it has only
one color. The laser is a perfect example of this kind of
light. The students put each filter on the red laser and
aim it at the palms of their hands. The red filter lets the
most light through and shines a bright beam on the
palm. The blue will shine a faint light. The green, which
showed the least red using the diffraction grating and
the flashlight, will essentially block the red light and no
red light should appear to shine on their palms.
Optics with Light and Color
59
Additive Color Model and Vision
Evaluate
INVESTIGATION
B2
Name ____________________________________________ Date ________________________
1. In this lab, you mixed colored lights together to get light that appears different. Match the combination
on the left with the resulting color on the right.
green + red
magenta/pink
blue + green
yellow
blue + red
white
blue + red + green
cyan/light blue
green + red make yellow
blue + green make cyan/light blue
blue + red make magenta/pink
blue + red + green make white
2. What is the process of mixing colored light called?
Mixing primary colors of light to achieve all the colors of visible light
is called the additive color model.
3. What are the cell types in human eyes that perceive color?
Red cone cells, green cone cells, and blue cone cells.
4. Describe two ways we might perceive light in the color cyan (light blue)?
One way is for the color to be from a single source. It enters the eye
and is detected by our blue and green cone cells. Another way is
for green and blue lights to combine in the same area. They both
enter the eye together and are detected by the blue and green cone
cells as coming from the same area, creating the perception of cyan.
5. A diffraction grating is a good way to disperse light. Explain how this might confirm what is happening
when you add different colors of light together to make white light.
When we look at white light through the diffraction grating glasses,
we can see that the light is made up of many colors including red,
orange, yellow, green and blue. If we were to combine light in these
colors, we would get a white or whitish light.
Copyright © CPO Science
Can be duplicated for classroom use
B2 Additive Color Model and Vision
Optics with Light and Color
WRAPPING UP
Have students reflect on what they learned
from the investigation by answering the
following question:
Write a paragraph that summarizes how we
perceive the colors of light. Use vocabulary you
learned in this investigation in your paragraph.
60 Notes and Reflections