Re: [Phys-l] refraction question

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

Of course John Denker is right that color is subjective.  But we have now
defined color to be related to the frequency or wavelength.  But this is for
light in a vacuum.  So color is a perfectly good reference and is used in
many good scientific articles.

Of course the light inside your eye has a slower speed so the correlation
between wavelength inside the eye and the color has changed.  The color
really depends on the frequency which relates to the resonant absorption of
specific molecules inside the cones in your eye.  But this is a macroscopic
description.  The microscopic would of course talk about the energy and the
frequency.

Inside the glass a given frequency photon would have the same frequency
inside your eye.  Students who ask this question may not understand that the
speed and wavelength are completely determined by the frequency and medium,
and that the frequency does not change.

One of the things that emerges from PER is that students tend to consider
things that are irrelevant rather than just looking at the relevant
variables.  So for example students tend to think that the force a spring
exerts is determined by what it is attached to rather than the stretch.
Students need to be helped to relate variables correctly, and this is
usually not possible if they are just presented with equations.

Now perhaps they are thinking that instead of your eye being inside the
glass, just the nerves and cones are inside the glass encased by glass.  But
as soon as the light goes into the nerve it has the same speed as it does in
the nerve in your physical eye.  So suppose the water in your optic nerves
and cones were replaced by glass.  If the molecular resonance does not
change then the color would still be the same.

The color you perceive is related to specific dyes that are in the cones.
And one of the dyes (The blue one?) comes in one of two different
genetically inherited molecules.  In addition the density of cones and
absence or partial absence of one of the 3 cone types makes color perception
quite different between individuals.  Then of course "color constancy" of
colored materials makes the definition of subjective color in terms of just
the ratios of different spectral frequencies meaningless.  But despite this
there are tables of color vs frequency which are compiled under controlled
conditions.  So color is not a good substitute for frequency, but it is
useful in descriptions to help build student understanding.

Unfortunately most textbooks completely ignore some of the facts about how
the eye works.  In particular they pretend that your subjective experience
of color is the same as everyone else's.  And they completely ignore color
constancy which is readily observable.  As I recall in the late 60s Edwin
Land even demonstrated this at an APS meeting.  Once you understand the high
level of subjectivity of color, you tend to ask different questions.

So is the question a variable confusion, or a misunderstanding of color vs
frequency, ...  What thought process brings this up?  It is a good question,
because at least they are thinking and asking it.  What is the difference
between students who ask it, and students who don't?  Are the students who
don't completely incurious, or do they go through a reasonable chain of
reasoning and decide this is either not a valid question, or easily
resolved.  Good teaching involves asking oneself the pedagogical questions
as well as the physics questions.

John M. Clement
Houston, TX

> I would tend to define the color of the light to be related to the energy
> each photon of light "delivers" to your retina.  Since the energy of the
> photon (hf) depends on the frequency, the color of the light remains the
> same even if the wavelength and velocity change --- as long as the
> frequency does not.  A good question that I have had students ask me
> involves the color the light laser light would look if, instead of
> observing it from outside of the glass as we nearly always do, you were
> inside the glass.  I suspect that, even though the wavelength and speed
> were different, you would see the same color.