Re: [Phys-l] "Unlearning"

["John Clement" <clement@hal-pc.org>]

>
I never saw the statement that wavelengths can be combined, only that color
can be combined.  This is a nice experiment that can be done using a
simulation and a computer.  Students can readily find that color mixing
works and figure out what colors can be combined.  So it is a good
experimental exercise.

While it is true that what the eye sees using natural light does not line up
with the simple experiment.  But the simple experiment does show how one can
engineer colors for TV and other displays.  The 3 color model works fairly
well for film and TV.  It just fails to work for items under natural
illumination.  This can be demonstrated using colored lights and a Mondrian
of different color papers.

So I would say it is useful as another opportunity for students to learn how
experiments work, and that a given model may only work for specific
conditions.

Actually the circuitry that produces color constancy is now fairly well
known.  A given cone is stimulated by the appropriate range of wavelengths,
but nearby cones that sense the same color suppress the stimulation.  So a
uniform field of color will cause no stimulation of a cone in the middle of
the field, and only cones at the edge of the field are stimulated.  In other
words your eye converts the world to line drawings with only the boundaries
being sensed.  Then your mind fills in the uniform fields.

And there are two genetically different sets of cones.  So the range of
sensitivity to wavelengths actually varies from one person to the next.
Factor in the varying density of cones, and random variations in the brain,
one wonders why colors can be perceived as uniformly as they are.  Of course
the in between primary colors are perceived differently so people will ague
over whether something is blue or green, and each one is right with respect
to their sensations.

Also the color model is interesting to students, plus it is not well taught
elsewhere.  So why not do it in physics?  Incidentally the 3 primaries
taught in art class were credited to Goethe, better known for his poetry,
and maligned for his lack of appreciation of music.  He insulted Berlioz!

John M. Clement
Houston, TX


>   My "favorite" piece of false information that is regularly taught in
> physics classes is that color equals wavelength, and that wavelengths
> can be combined through Additive Color Mixing - that Red+Green makes
> Yellow, Blue+Green makes Cyan, etc., like on a TV screen.
>
> This is in no way related to Physics - if you look at the wavelengths
> involved, it wouldn't work out, even in a non-linear medium.
>
> This is all about Biology. And not just general biology, Human biology.
> Unlike the ear, which can pick out thousands of individual frequencies
> and combinations of these frequencies (such as musical chords and more
> complex sounds like voices), the retina only responds to three main
> bands of wavelength, and produces color through a complex and not
> completely understood interaction over the entire visual field. (What
> wavelength is Silver? How about Pink? Brown? - see color is not just
> about wavelength).
>
> As far as physics is concerned, Red plus Green simply makes Red plus
> Green. EM radiation of multiple wavelengths can pass through the same
> space and remain distinct (otherwise we'd have only one radio station to
> listen to). Most objects that look Yellow are actually reflecting Red
> and Green. Color Addition does not belong in physics books! It is
> cognitively at odds with other basic information that has been taught.
>
> As far as refraction goes, it makes no sense to describe a light wave as
> having a width, where you can grab one end and slow it down, while the
> other end rotates around as if it were a stick. We could simply use the
> shortest time of travel, or just teach it through simple observation and
> measurement, and then give Snell's law to quantify the relationship
> without a real explanation.