Re: [Phys-l] experiments with dyes (was: happy equinox)

["Michael Edmiston" <edmiston@bluffton.edu>]

Michael Porter suggested (see below) that magenta and cyan might be 
misconstrued.  AFAICT, they are almost always misconstrued.

A long time ago (and I don't know the reference right now) a Physics Teacher 
article suggested which theatrical gels under the trade names of Roscolene 
and Roscolux did the best job of color mixing using both the additive system 
(red, green, blue) and using the subtractive system (yellow, cyan, magenta). 
I purchased large sheets of the suggested gels from a theatrical supply 
dealer many years ago and I still use them.  I cut some of them into 2" X 2" 
squares that I can distribute to a class.

I have tested the filters and they are not bad.  My judgment of good/bad is 
done by using a spectrophotometer, and looking for the following...

The red filter transmission should fall off rapidly below approximately 600 
nm.
The green filter transmission should fall off rapidly above approximately 
560 nm, and below approximately 500.
The blue filter transmission should fall off rapidly above approximately 470 
nm.

If you have filters like this you can use three adjustable-brightness light 
sources, one for each filter, and you can overlap them and adjust them to 
get white where they all overlap, and you can get yellow where the red and 
green overlap, cyan where the green and blue overlap, and magenta where the 
blue and red overlap.  This is another way to judge them without using a 
spectrophotometer.  My set works beautifully in this manner.

***However*** when practically any novice looks at these for the first time, 
they accept the green as green, but they say the blue is violet, and they 
say the red is way too dark to be an "ordinary" red.

For a good set of subtractive primaries, as determined with a 
spectrophotometer, the filter should do a good job of blocking the 
complementary additive primary, and have good transmission throughout the 
other two additive primary regions.  For example, the cyan filter should 
totally block red (block from 600 on up) and transmit equally well through 
the blue and green regions (from 400 to about 560.)  Likewise, yellow should 
block blue (everything below about 500) and transmit everything above that. 
Magenta should wipe out the middle (block the green region from about 500 to 
560.)

If you have these, you can use them subtractively (hold two together) to 
show yellow and cyan make green, yellow and magenta make red, and cyan and 
magenta make blue.

***However*** novices looking at these for the first always claim the cyan 
filter is blue.  They also claim the magenta filter is red, although they 
admit is perhaps more of a fuchsia-red.  This is the misconception Michael 
Porter mentioned.

If you have both the subtractive set and the additive set you can also show 
that holding up the blue and the yellow together do not make green, but make 
black.  ***However*** novices will claim getting black from yellow and blue 
(when added subtractively) is cheating because they refuse to admit the blue 
filter is blue.  They only want to accept the cyan filter as blue (albeit 
"sky blue" according to some) and claim the blue filter is something else 
(usually violet).  The best way I have been able to convince them that cyan 
is not blue, is to project a bright spectrum on a screen using a slide 
projector with a slit and a diffraction grating, and having them look at the 
spectrum through the cyan filter, and seeing that the cyan filter passes 
every bit as much green as it does blue.  I ask them why they are calling it 
blue when it is obvious they could just as easily call it green.  At this 
point some see the light (sorry), but others continue to think I am somehow 
tricking them... that the cyan filter is indeed blue, but my demonstration 
that it passes as much green as blue is some sort of magic trick.

Another good "trick" to do with a yellow and cyan filter is to combine them 
additively.  Put each in separate slide projectors.  Power at least one 
slide projector with a variac so you can adjust the brightness (you usually 
need to dim the yellow one).  Before class, adjust the brightness so the 
addition of the two produces a good white color (with no yellow cast or cyan 
cast).  Then aim the slide projectors so they don't overlap.  When the class 
comes in, they can see the yellow and cyan projected on the screen.  When 
class starts ask them what color will result of you move one of the 
projectors so that the two colored regions will overlap.  Virtually everyone 
will say the overlap will be green.  When you push the two colors together 
to produce white they can barely believe it.  That's how I generally start 
my first class session about color.

For the nitpickers, it is clear that the full story of color perception and 
the ability to create any hue, saturation, etc. is more complicated than 
simple additive or subtractive mixing with three "primaries."  However, the 
three-primary model of additive and subtractive mixing is a good place to 
start.  I think it is difficult to move very far toward understanding color 
until the three-primary model is understood.  Then you can examine the 
limitations and move beyond it.

Summary... obtaining a good set of additive filters and subtractive filters 
is a very useful way to start...  but you still need to be prepared for a 
large number of disbelievers, and if you have time you should try to move a 
little bit beyond it.

Michael D. Edmiston, Ph.D.
Professor of Chemistry and Physics
Bluffton University
1 University Drive
Bluffton, OH 45817
419.358.3270
edmiston@bluffton.edu


--------------------------------------------------
From: "Michael Porter" <listmoe@gmail.com>
Sent: Monday, March 24, 2008 12:12 PM
To: "Forum for Physics Educators" <phys-l@carnot.physics.buffalo.edu>
Subject: Re: [Phys-l] experiments with dyes (was: happy equinox)

<snip>

> I always thought that a casual glance at the true subtractive
> primaries might lead to the misconception -- cyan looks blue, magenta
> is kind of red...