Re: REFRACTION - REFLECTION

[herbgottlieb@juno.com]

On Thu, 22 Jan 1998 15:19:01 -0600 "Dewey Dykstra, Jr."
<dykstrad@bsumail.idbsu.edu> writes:
> > Refraction is fundamentally a *wave* behavior and, as such, any 
> analogy using particle-type models such as lawn mowers, tricycles, etc.
have 
> nothing to do with the effect.  If the messy details of the wave nature
of the 
> phenomenon are to be suppressed because they are too advanced for a
lower level 
> class then an appeal to Fermat's principle can be made.  This is because

> any wave propagation phenomena obeys Fermat's principle in the short 
> wavelength limit and then the analysis can be done entirely in terms of
rays.  
> In this case refraction is an automatic result of changing the speed of
the 
> rays in going from one fixed place to another fixed place while
minimizing 
> the time of travel for those rays.
> > David Bowman
> > dbowman@gtc.georgetown.ky.us

Ok, OK, but how do you explain this to a student when SHE asks:

"When a beam of light enters the surface of water at an angle of 
25 degrees with the normal why doesn't it continue at the same 
angle after it enters the water?  Does Fermat's principle 
explain why the wavefront must always perpendicular  to the direction 
of travel? 

Herb Gottlieb from New York City 
(Where wavefronts have learned that they must ALWAYS be
perpendicular to the direction of wave propagation)