Re: [Phys-l] refraction question

[Brian Whatcott <betwys1@sbcglobal.net>]

Anthony Lapinski wrote:
> When sound waves refract, the frequency always remains constant. Thus, if
> the velocity decreases, the wavelength also decreases. This same idea
> holds for light. When light refracts, the frequency remains constant. But
> how does this relate to the COLOR of the light? Does the color depend on
> frequency or wavelength?
>
> The reason I ask is that suppose you shine red light (say, 680 nm) from
> air into water. Since n = 1.33, both the velocity and wavelength will
> decrease by this factor. Thus, (680 nm)/1.33 = 511 nm. This is the
> wavelength of green light! We've probably all done this demo with a red
> laser, and the beam remains red. So color depends on frequency. In class I
> usually say that color depends on wavelength. Lasers are rated by their
> wavelength. Instead, should lasers -- like tuning forks -- be rated by
> their frequency since this quantity never changes?
>
> _
I reviewed all the responses that this note of yours provoked.
I was struck that possibly the best was also the first to arrive
here - the one from Curtis.

The human eye is found to provide single quantum sensitivity, though at 
low efficiency -
and this only applies to rod sensing which is not color selective.

The human color sense is a nicely processed  effect of the output of 
three cone
sensor types,   which are usually evaluated as ratios, so that a minor 
difference
in sensitivity to incident illumination by one of the types can be 
sensed as a color shift
owing to the changing ratio of the adjacent spectral responses.

The light sensor has the unexpected property that it is a repetitive 
firing output,
whose periodic time INCREASES with increasing illumination.
Given that the cone sensor accumulates energy for its color sensing 
molecular response,
it's not unrealistic to suggest that the energy increments depend on the
frequency of the input.
I liked the assertion that wavelength as such is not relevant at the 
point of sensing.
This would contrast with the aural sensory mode - which maps acoustic 
frequencies
to position in the sensory path - so that a case could be made there
for wavelength sensing.

What are we to make then, of the diffraction grating, which directly 
transduces
wavelength to transmission angle? it would be reasonable to suppose 
(and easily tested
experimentally by means of  waterproof sensors submerged in the 
experimental fluid)
that the transmission angle DOES vary with the fluid in which a grating 
is immersed.

I conclude that simply because some emitters are rated by their 
wavelength in
air or vacuum,   this is not sufficient to make their wavelength the 
controlling factor
for sensory perception of color, and neither is the periodic time of the 
light
controlling either.

The time scale for light signaling is so much slower (frequency 
modulation of a
pulsed neural signal happens at a pulse rate measurable in kiloHertz.)

Brian W