Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: [Phys-l] refraction question



On 04/23/2010 04:49 AM, 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?

Color is best defined a _perception_. It depends on the
properties of the light at the time it hits the retina.
It does not depend on history; it does not depend on
the list of media that the light might have passed through
on its way.

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!

You can confirm and demonstrate the wavelength by
sticking a diffraction grating in the water.

We've probably all done this demo with a red
laser, and the beam remains red. So color depends on frequency.

This strikes me as mostly a philosophy and metaphysics
question, not a physics question. It hinges on what
you mean by "seeing" and what you mean by the "same"
light: is the light that hits your eye the "same" as
the light that passed through the water tank?

To illustrate what I mean: suppose you "see" some light
that came from a distant galaxy, and has undergone a
significant (z=2) cosmological and/or gravitational
redshift since it was emitted. Is the light that hits
your eye the "same" light that was emitted? In some
ways it is the same ... but both the frequency and
wavelength are different.

As a contrasting example, if you redefine "color" to
be based on what happens when the light hits a diffraction
grating, then the color of a beam of light in water is
different from the "same" beam in air.

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?

Saying "never changes" is too strong.
-- There are some things that change the wavelength
and some that don't.
-- There are some things that change the frequency
and some that don't.
-- There are some situations where wavelength is of
interest to the end-user.
-- There are some situations where frequency is of
interest to the end-user.

Even if we restrict attention to changes in the
refractive index of the local medium, if the light
goes into a medium and back out again, there is no
net change in wavelength (although there commonly
is a net change in the wavevector). The thin-lens
equation (for example) is based on this assumption.
So it is going too far to say that when light is
refracted its wavelength changes.

Note: The wavelength specification of the laser is
unambiguous, because everybody knows it refers to
the wavelength _in air_.

Radio terminology is overcomplete: "shortwave radio"
is pretty much synonymous with the "HF band".

Eyeballing the perceived color of light scattered
inside a tank of water is not a good way to ascertain
the wavelength _in water_ of the light.

My suggestion: For serious physics work, don't talk
about the intrinsic "color" of the light. Color is
not a precise concept and cannot be made precise.
-- If you mean frequency, say frequency.
-- If you mean wavelength, say wavelength.
-- If you mean wavevector, say wavevector.