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Re: [Phys-l] refraction question



A complete treatment for the origin of the refractive index is pretty complicated, and likely not easily explainable to students who haven't had a lot of complex analysis (unless some heuristics are allowed). James and Griffiths (yeah, that Griffiths) had a nice write-up in 1992, in

Why the speed of light is reduced in a transparent medium
American Journal of Physics -- April 1992 -- Volume 60, Issue 4, pp.
309-313



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________________________________
From: "Edmiston, Mike" <edmiston@bluffton.edu>
To: Forum for Physics Educators <phys-l@carnot.physics.buffalo.edu>
Sent: Fri, April 23, 2010 7:43:57 AM
Subject: Re: [Phys-l] refraction question

In response the this question, daryl@darylscience.com gave an explanation based upon the light being continuously absorbed and reemitted. I've seen this explanation in several physical science textbooks aimed at both the high school level and also the conceptual physics level in college.

I don't like this explanation very well. Do the rest of you accept this? This explanation seems to promote the idea that the interaction between the E&M radiation and the atoms in the medium involve complete absorption that excites an electron to a higher quantum state, then reemission. This is the description we use for spectroscopic line absorption and line emission. If that is the model being proffered for explaining the slower velocity of light in a medium other than vacuum, it seems to me we should be protesting.

Michael D. Edmiston, Ph.D.
Professor of Chemistry and Physics
Bluffton University
Bluffton, OH 45817
(419)-358-3270
edmiston@bluffton.edu



-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-bounces@carnot.physics.buffalo.edu] On Behalf Of daryl@darylscience.com
Sent: Friday, April 23, 2010 8:23 AM
To: Forum for Physics Educators
Cc: phys-l@carnot.physics.buffalo.edu
Subject: Re: [Phys-l] refraction question

Hi, Anthony. I'll take a stab at this.

The photons does not change wavelength while traveling through glass or water. That is an analogy we use to describe the air-glass interface and should not be perceived as a real model. Follow one red photon. It will be absorbed by an electron in the first glass molecule it bangs into. This electron will "jump" to a higher energy state and come right back down. Thereby, it emits an identical red photon. However, while the photon is "in existence", it is traveling at 'c' through the empty space between the glass molecules and has a typical red frequency and red wavelength. You cannot change the characteristic frequency of a light wave by passing it through a typical transparent material.

The perceived slow down is simply that the red light has to make so many stops along the way. It gets absorbed and re-emitted millions of times. I tell my kids it is like traveling on I-95 for a family trip at 70MPH for an entire trip, but making several pit stops for potty and drinks. The AVERAGE speed of the path decreases, not the instantaneous speed of the traveler at any given driving time. So, the last red photon spit out by an electron has the same properties as the original photon that entered the other side of the glass. Or water. The slowdown and wavelength shift are simply the culmination of traveling at 'c' and making a Bajillion pitstops along the way.

I can't remember which text author, maybe Hewitt, had a nice description and diagram of this years ago when texts were still made from paper...

Hope this helps. Better yet, hope this is sorta right...

-DT


Anthony Lapinski <Anthony_Lapinski@pds.org> 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?

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_______________________________________________
Forum for Physics Educators
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