Color Mixing
With Light
Overview
Mixing colors of light is very di erent than mixing colors of pigment, dye, or ink. If you mix red and
green paint, you end up with brown. If you mix red and green light, you get yellow light! How can
this be?
Theory
Our eyes determine the myriad of colors we see in the world, so let’s take a closer look at what is
happening inside the eye. At the back of the eye is a thin layer called the retina. There are millions of
special photoreceptor (light-sensing) cells in the retina called rods and cones. When light enters the
eye and reaches the retina, the rods and cones detect the light waves and send signals containing
information about the light to the brain to be interpreted. Rods are responsible for night vision as
they work in very low levels of light. Cones need a lot more light and are responsible for our color
vision. Humans have three types of cones: red, blue, and green. Even though they are labeled by the
color they are most sensitive to, the cones overlap and detect other colors of light resulting in the
many colors we see.
The three primary colors for light mixing are red, blue, and green and that’s because we have those
three types of cones! When we add the three primary colors of light equally together we get white.
The three primary colors can be added to each other in di erent proportions to create all other
colors.
Secondary colors are those created when two primary colors of light are added together. This is
called additive color mixing. Two lights of di erent wavelengths are mixed and they create a new
color, and all of the wavelengths reach our eyes. The secondary colors of light are cyan, magenta,
and yellow. When we mix blue and green light, we get cyan. Red and blue light creates magenta, and
green and red light make yellow.
If we place a secondary color lter over a white light we get a subtractive process. The lter absorbs
one of the primary color wavelengths, so one primary color is subtracted from the spectrum. If we
use a cyan lter, red would disappear. A magenta lter would subtract green, and a yellow lter
would remove blue.
Doing the activity Necessary materials:
This is designed to be an exploratory demo of what • Three LED ashlights: one red, one blue,
and one green
happens when you mix colors of light. The
following directions are suggestions. • A white surface (ceiling, wall, piece of
paper or fabric)
1. Have 3 students come up and hand each one of • A darkened room
them a primary color ashlight (red, blue,
If you would like to use cyan, magenta, and
green).
yellow gel lters, you can order them from
2. Have them shine the 3 lights separately on the Amazon. You can also order rainbow glasses
white surface, but don’t have them combine from Amazon.
the light yet.
3. Ask your class to predict what would happen if you mixed the red light with the blue light. Have
those two students demonstrate. You may have to clarify the color as magenta. Continue with
blue and green making cyan, and red and green making yellow.
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4. Now ask them to predict what would happen if they combined all three lights together. Have the
students with the ashlights demonstrate.
5. Discuss.
Extension 1
Dim the lights, and have the students use the di erent ashlights around the classroom. Do the
colors of clothing and other objects change, when they shine a certain color of light on them.
Normally we see things in sunlight or arti cial white light, which are composed of all colors of light.
An object appears a certain color because it re ects that color. Do they look di erent when they are
under a red, blue, or green light?
Extension 2
For this activity, you will need cyan, magenta, and yellow lters, a white incandescent light, and
rainbow glasses.
Dim the lights and have students look at the white light with their rainbow glasses. Now place one of
the secondary lters (cyan, magenta, yellow) over the light. What primary color was absorbed by the
lter and was subtracted from the spectrum? Continue trying the di erent lters. What happens if
you put two lters over the white light? What happens if you use all three?
Summing up
This activity was designed to be an exploration of what happens when you mix colors of light.
Students will experience the three primary colors of light and will discuss with their teacher how and
why their eyes perceive those colors due to the cones in their eyes. They will be able to create
secondary colors and white light.
For more information
Little Shop of Physics: https://www.lsop.colostate.edu
Colorado State University College of Natural Sciences: https://www.natsci.colostate.edu
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