Re: [Phys-l] refraction question

["John Clement" <clement@hal-pc.org>]

> So is D not the proper answer for a multiple-choice question like this?
>
> If a laser beam passes from air into water, then its velocity _____ and
> its wavelength _____.
>
> A. increases; increases
> B. increases; decreases
> C. decreases; increases
> D. decreases; decreases
This discussion is how do you model something?  At the macroscopic level you
look at the macroscopic things you can measure, so you conclude that as the
speed decreases the wavelength also shortens.  But if you are looking at the
microscopic level you use a different model.

The macroscopic model works very well at interpreting and predicting what
you see in the introductory lab, or how lenses, mirrors, gratings,... work.
But it can not tell you how to make a material with higher or lower index of
refraction.  So there you need to go to a microscopic model.  The big
problems are where neither model can be used successfully, such as the
region between micro and macro.

Engineers and physicists still use models which sometimes may be considered
invalid but useful.  And similarly one can not present to students a model
which is far beyond what they have studied, but one can give them small
pieces of the advanced models to help them gain understanding.

And this MC question is the typical type of thing that textbooks use to test
understanding, but often just tests memorization.

So how would you write an intro book to explain the photon interactions
which underlie the macroscopic description?  Notice that you can't use full
"expert" arguments here.

John M. Clement
Houston, TX