We most often sample both light and sound waves with plane-wave sensors (film, CCDs, and retinas for light; eardrums, microphones, and other transducers for sound). These sensors, for all practical purposes, don't have extent in the wavevector direction, but perpendicular to it (along the wavefronts). That means that the detectors have no idea what the wavelength is impinging upon them.
The wavelength is sort of an "extended" phenomenon, rather than a "local" one, especially when talking about an individual cone cell, or a silver halide crystal, or -- especially in the case of sound -- an eardrum. These sensors don't care about what waves are doing elsewhere; it's what they're doing at the sensor that matters.
And I like your example of a laser going through water (and our eyes are water, for all intents and purposes). One doesn't have to use a camera which detects wavelengths 1.3 times shorter when diving.
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As a species, we are forever sticking our fingers into the electric socket of the Universe to see what'll happen next. It's a trait that'll either save us or kill us, but by god it's what makes us human beings. I'd rather be in the company of people who look at Mars than people who contemplate humanity's navel -- other worlds are better than fluff. ~~Sir Terence David John Pratchett
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From: Anthony Lapinski <Anthony_Lapinski@pds.org>
To: phys-l@carnot.physics.buffalo.edu
Sent: Fri, April 23, 2010 5:49:49 AM
Subject: [Phys-l] refraction question
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?