* * * Brian said... * * *
From a set of kid's acrylics, I took a brushful of green
(there was only one green) and a brushful of blue
(there was only one blue) and I mixed them on
brilliant white cartridge paper taken from my printer.
And I got a color that I used to call 'turquoise'.
Not exactly cyan, I don't think but far, far from black.
* * *
I don't doubt this. I believe it means Brian's blue pigment
reflects/transmits well above 450 nm and his green
pigment reflects/transmits well below 500 nm. Remember
the original question in the physics book said "pure blue"
and "pure green" without explaining what "pure" means.
This is not unusual. I have done a lot of playing around
with colored filters as well as paints and dyes. I have the
equipment to record transmitted spectrums as well as
reflected spectrums. A lot of objects people call "blue"
extend way into the green, and vice-versa. The same
happens with "green objects" extending into red and
"red objects" extending into green.
Without using spectrophotometers, here are some
interesting experiments you can do, and a whole class
can do these things. I've even done these things
at the 1st-grade level at the local school.
Equipment... obtain a collection of different colored
filters, e.g. theatrical gelatins used in stage lighting.
Also obtain some really bright diffraction grating
material. Obtain a 35-mm slide projector. Cut a
2"x2" stiff paper "slide" and make a slit in it that is about
1/8-inch wide.
(1) Put the slit in the slide projector with the slit
vertical. Project the slit onto a screen (best) or white
board (okay). Focus the slit. Darken the room as
much as possible.
Tape a piece of grating film over the lens and orient the
grating so spectrums appear to the right and left
of the slit. Re-orient the slide projector so one of the
spectrums is on the screen.
The spectrum and your eyes will now be a color
analyzer for the colored filters.
Hold a colored filter in front of the projector lens.
Observe the spectrum, particularly note which
parts have been blocked and which parts are
coming through the filter.
Many, many filters average students call "blue"
are easily seen to transmit significant green, and vice-versa.
It is difficult to find a "pure blue" filter, and when you do, a lot
of students will say... "That's not blue, it's violet."
This is really interesting because when they view the spectrum
alone, they all agree which part is blue, which part is green,
and they agree cyan is in the middle. But... when you find
a filter that lets only the blue light through, the students will
call it violet. They have been accustomed to "blue" that
is really more cyan than "true blue."
This "analyzer" is particularly great for complementary
colors. A yellow filter wipes out the blue portion of the
spectrum. A magenta filter wipes out the green portion,
and cyan filter wipes out the red portion. This is very
easily observed and students quickly realize that
magenta is "anti-green" and so forth. I try to use as many
descriptive words as possible. The magenta filter is
anti-green, it eats green, it absorbs green, it blocks green,
it is not-green, it is the complement of green.
(2) Cut out squares of various theatrical gels and distribute
these to the class. I distribute a primary blue, primary green,
and primary red. These are fairly dark because their
transmission bands are fairly narrow. I also distribute
cyan, magenta, yellow. I give each student a 2"x2"
square of each color.
[Note: I am at home and cannot give you the
manufacturer's numbers for the gels I use. I can
post that information later.]
Have the students look at various objects through
one of the filters.
For example, if you use a magenta filter, then any
object that is colored green will appear black
when viewed through a magenta filter.
For this current discussion, use a yellow filter
to view blue objects. If the blue pigment is
truly blue (with little green) then the object will
appear black when viewed through a yellow
filter. If the object does not appear black, then
significant green light is coming from that "blue"
object.
Once you have the appropriate filters there are
lot's of neat and fairly education things you can
do.
Michael D. Edmiston, Ph.D.
Professor of Physics and Chemistry
Bluffton College
Bluffton, OH 45817
(419)-358-3270
edmiston@bluffton.edu