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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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