I am bothered by some of the things Vickie Frohne has said.
She said, "RYB works very nicely for kindergarten paints and food dyes.
These paints and dyes ARE red and blue, NOT mis-named magenta or cyan.
Kids and cooks easily make orange with RY, green with YB, and purple
with RB."
I disagree. I believe if you mix "blue and yellow" paint and end up
with green, the paint you used was not blue, but was a blue-green mix,
i.e. cyan.
Of course it depends on the definition of blue. Although Vickie goes on
to describe that different people have different color perceptions, I
think we can't just leave it as a free-for-all. There are easy
scientific ways to demonstrate that what some people call blue is
usually just as much green as blue.
Before describing some easy ways to show this, let me first qualify
something that others have pointed out (I think Larry Woolf was one of
them) that there is a difference between subtractive color mixing done
with dyes versus that done with pigments. The distinction being made
here is that pigments are macroscopic particles not dissolved in the
vehicle, and dyes are molecular sized because they are dissolved in the
vehicle. The macroscopic nature of pigments does complicate things a
bit. I ignore that in what I say below.
I have used a fiberoptic spectrophotometer with a reflectance probe to
measure the spectral reflectance of paint pigments. If you take a
pigment that people are calling blue, but it yields green when mixed
with yellow pigment, the reflected spectrum of that "blue" pigment runs
way into the green region of the spectrum. In other words, that "blue"
pigment could just as likely be called green as blue. Of course some
pigments are more blue and less green whereas some are more green and
less blue. But the pigment has to be a mix of green and blue if you
perceive green when the pigment is mixed with yellow.
Thus, the "blue" pigment that is capable of producing green when mixed
with yellow really is mislabeled cyan, contrary to Vickie's statement.
If you perform subtractive mixing with blue pigment that does not show
reflectance into the green region of the spectrum (I would call this
blue or true blue) and mix it with yellow, you get black.
In a favorite classroom demonstration I use a slide projector with a
vertical slit in the slide position, and a diffraction grating or prism
in front of the lens. The goal is to get a nice bright spectrum
projected onto a screen in a darkened classroom. I then use theatrical
gels for examining color and subtractive mixing.
You can use one gel and place it in front of the projector lens, or you
can have one gel per student and have them observe the spectrum through
the gel.
Have them look at the spectrum through a yellow gel. Advanced students
tend to say it transmits the green and red portions of the spectrum.
Younger kids are a bit more fun and might say things like, "Wow, it like
totally wipes out the blue." Of course these are both correct. We can
view the yellow as transmitting red and green, or as absorbing the blue.
Young kids like to say the yellow eats the blue. It is very obvious and
even striking.
Next, give them a gel that most people call blue, but it is really cyan.
View the spectrum through it, and it eats the red, but lets through just
as much green as blue. People who would swear it is what they call
blue, readily admit is it just as much green as blue when viewed this
way. It is then obvious that when you look through both the "blue" and
yellow, the "blue" (which is really cyan) eats the red and the yellow
eats the blue, leaving the green.
It is difficult to find "true blue;" that is, blue gels that neither
transmit any green nor any red. I do have some that are pretty close.
When people view the spectrum through these, they admit that it lets the
blue light through but not the green or red. They admit that it is
"just blue." However, when you ask them to describe the color of the
gel, some say it is violet, whereas others say it is really really
really dark blue.
Anyway, when you combine this true blue with the yellow, the combination
is very black.
If you have not projected a spectrum and then observed it through
colored filters, you really need to do it. It leaves a very strong
impression on students of all ages about how subtractive color mixing
works.
Michael D. Edmiston, Ph.D.
Professor of Chemistry and Physics
Bluffton University
Bluffton, OH 45817
(419)-358-3270
edmiston@bluffton.edu
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