When light enters the eye the wavelength becomes dependent on the index of refraction of the fluid in the eye - regardless of what media the light went through previously. The fact that you see the light as red when it passes through water does not, per se, prove that perceived color depends on frequency instead of wavelength.
Bob at PC
-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-bounces@carnot.physics.buffalo.edu] On Behalf Of Anthony Lapinski
Sent: Friday, April 23, 2010 7:50 AM
To: phys-l@carnot.physics.buffalo.edu
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?