2/21/2021 Physics Tutorial: Color Subtraction
Color Subtraction
The Electromagnetic and Visible Spectra
Visible Light and the Eye's Response
Light Absorption, Reflection, and Transmission
Color Addition
Color Subtraction
Blue Skies and Red Sunsets
The previous lesson focused on the principles of color addition.
These principles govern the perceived color resulting from the
mixing of different colors of light. Principles of color addition have
important applications to color television, color computer monitors
and on-stage lighting at the theaters. Each of these applications
involves the mixing or addition of colors of light to produce a
desired appearance. Our understanding of color perception would
not be complete without an understanding of the principles of
color subtraction. In this part of Lesson 2, we will learn how
materials that have been permeated by specific pigments will selectively absorb specific frequencies of
light in order to produce a desired appearance.
We have already learned that materials contain atoms that are capable of selectively absorbing one or
more frequencies of light. Consider a shirt made of a material that is capable of absorbing blue light.
Such a material will absorb blue light (if blue light shines upon it) and reflect the other frequencies of
the visible spectrum. What appearance will such a shirt have if illuminated with white light and how
can we account for its appearance? To answer this question (and any other similar question), we will
rely on our understanding of the three primary colors of light (red, green and blue) and the three
secondary colors of light (magenta, yellow and cyan).
The Process of Color Subtraction
To begin, consider white light to consist of the three primary colors of light -
red, green and blue. If white light is shining on a shirt, then red, green and
blue light is shining on the shirt. If the shirt absorbs blue light, then only red
and green light will be reflected from the shirt. So while red, green and blue
light shine upon the shirt, only red and green light will reflect from it. Red and
green light striking your eye always gives the appearance of yellow; for this
reason, the shirt will appear yellow. This discussion illustrates the process of color subtraction. In
this process, the ultimate color appearance of an object is determined by beginning with a single color
or mixture of colors and identifying which color or colors of light are subtracted from the original set.
The process is depicted visually by diagram at the right. Furthermore, the process is depicted in terms
of an equation in the space below.
W - B = (R + G + B) - B = R + G = Y
Now suppose that cyan light is shining on the same shirt - a shirt made of a
material that is capable of absorbing blue light. What appearance will such a
shirt have if illuminated with cyan light and how can we account for its
appearance? To answer this question, the process of color subtraction will be
applied once more. In this situation, we begin with only blue and green primary
colors of light (recall that cyan light consists of blue and green light). From this
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2/21/2021 Physics Tutorial: Color Subtraction
mixture, we must subtract blue light. After the subtractive process, only green light remains. Thus,
the shirt will appear green in the presence of cyan light. Observe the representation of this by the
diagram at the right and the equation below.
C - B = (G + B) - B = G
From these two examples, we can conclude that a shirt that looks yellow when white light shines
upon it will look green when cyan light shines upon it. This confuses
many students of physics, especially those who still believe that the color
of a shirt is in the shirt itself. This is the misconception that was targeted
earlier in Lesson 2 as we discussed how visible light interacts with matter
to produce color. In that part of Lesson 2, it was emphasized that the color
of an object does not reside in the object itself. Rather, the color is in the
light that shines upon the object and that ultimately becomes reflected or
transmitted to our eyes. Extending this conception of color to the above two scenarios, we would
reason that the shirt appears yellow if there is some red and green light shining upon it. Yellow light
is a combination of red and green light. A shirt appears yellow if it reflects red and green light to our
eyes. In order to reflect red and green light, these two primary colors of light must be present in the
incident light.
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