The Color Yellow

Question:  I teach physical science and am interested in color.  Is there 
any way for me to tell if something yellow is reflecting yellow wavelengths of
light or green and red, without analyzing the reflection spectrograph?  Can
you come up with a way that I can show them true yellow and an object that is
really reflecting red and green?
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I do not know much about color.  However, Physics Today had a
whole series of articles on color - from computer generation to our
perceptions of it, about 3 or 4 months ago.  Hopefully you can find it in
your
library, or get it from a University physicist somewhere nearby.  (1993) I
think you probably can do what you want by using simple colored filters on
your light source.  Filters usually will say what wavelengths they let pass
through.  Look at the object in white light, then in red, green and yellow.
I
am not sure what you should use as an object though - most things do not
reflect just a single color, but a wide range, with some shown much better
than others.

A. Smith
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You could just illuminate a white screen with two light sources,
one with a red filter and one with a green filter, and overlap the spots of
light; the overlap region should look yellowish, even though there is no
yellow light there (show this by viewing the overlap through a yellow filter;
it should look as dark as the red and green regions.)

R.C. Winther
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Neither respondent really answers the question.  The answer is yes, you can
tell whether the yellow that you are looking at is a true "yellow" (around
590 nm in wavelength), or whether the yellow is composed of red and green.

To do so without a spectrometer would require you to get a special yellow
filter.  This filter would have to transmits narrowly in the yellow, and
block both red and green.  If your yellow object is really "yellow", it
should still look yellow through the filter.  If your yellow object is
really emitting red and green wavelengths, it will look dark when viewed
through the yellow filter.

Conventional photographers gels do not do this.  They transmit broadly above
500 nm and are not selective.   It might be difficult to find such a filter.

It should be noted that light-emitting diodes are fairly monochromatic.
For example, LEDs that make yellow light do it fairly narrowly around 590
nm.   On the other hand, the yellow on a computer monitor must be a mixture
of red and green because those are the colors of the pixels.

Bob Erck
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