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[Physltest] [Phys-L] Re: Color (was LED mini-flashlight price break)



I've checked two food color dyes w/ my star. The red has a very sharp
cut off at 600 so has a bit of orange and all the red to 700 where my
eye fades out. The blue is interesting; it's only ~ 50 nm wide ~
centered on ~ 460 + it passes > 670, so it looks a bit purplish. I
haven't bothered w/ the orange and yellow, yet.

bc

Brian Whatcott wrote:

At 04:19 PM 1/28/2005, you wrote:


I am bothered by some of the things Vickie Frohne has said.
///
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.




At Michael's prompting, I took down the Project STAR spectrometer,
and the set of gels that Vickie suggested we buy, and did the test,
using a broad band halogen source..

I was not convinced with Michael's talk of 'blue is green' and 'true blue
makes black' for this reason - just as Vickie mentioned, each of the
Arbor Scientific gels comes with a transmission spectrogram.

So I am not exactly amazed when a low pass filter like LEE #101
"Yellow" which passes all visible frequencies below lambda= 510nm
(<5% transmission at 450 nm or shorter and >85% transmission
at 560 nm and longer) actually passes an array of frequencies that
we recognize as various colors.

When I looked at the spectral transmission of the LEE #113
"Magenta" gel, I see that it is a sharper low pass, with a secondary
bandpass in the violet.
(5% of 590nm, to 80% at 650nm and longer with a 15% pass bump
at 400 nm)

So I interpret my observations in this way: color gels have surprisingly
broad bandpasses and peak transmissions: it's hard to find a bandpass
under 50 nm wide.
Most will span 2 or 3 colors as we normally label them.

Could pigment flakes be much narrower in pass band?


Brian Whatcott Altus OK Eureka!



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