Re: Light slows down in glass?

["Richard W. Tarara" <rtarara@saintmarys.edu>]

I would suggest that it is important to stress that electromagnetic
radiation always propogates at the same speed (speed of light) but that when
light enters a transparent medium it is absorbed, reemitted, abosorbed
again, reemitted again etc. and that this process makes it _seem_ that the
light has moved slower in 'passing through' the glass.  It is the
_effective_ speed that is slower.  Having once said that, now we can go
ahead and analyze various phenomenon just as though the light actually did
slow down and get 'correct' and useful predictions.

I must admit that at one time I was confused by the contradictory statements
taken from different parts of the curriculum--"E&M radiation always moves at
the same speed (c)"  and  "light travels at different speeds through
different media".  Best, I think, to at least give lip-service to the
underlying reasons for this discrepancy.

Rick


-----Original Message-----
From: William Beaty <billb@eskimo.com>


> Hi all!  I'd like to get some comments on the following issue.  I've
> received more than one message like the following.
>
> > --- comments ---
> > The following paragraph, taken from your 'misconceptions' page is
> > incorrect:
> >
> > --- url ---
> > http://www.eskimo.com/~billb/miscon/miscon4.html#radio
> > PHYSICAL SCIENCE MISCONCEPTIONS IN K-6 TEXTBOOKS
> >
> > > LIGHT AND RADIO WAVES DO NOT ALWAYS TRAVEL AT "THE SPEED OF LIGHT"
> > > They only travel at 186,000 miles per second while in a perfect vacuum.
> > > Light travels a bit slower in the air, and it travels LOTS slower when
> > > inside glass.  Why does light bend when it enters glass at an angle?
> > > Because it SLOWS DOWN.  Why can a prism split white light into a
> > > spectrum? Because within the glass THE SPEED OF LIGHT IS DIFFERENT FOR
> > > DIFFERENT FREQUENCIES.  And while the numerical value for speed of
> > > light in a vacuum, "c," is very important in most facets of physics,
> > > there is no single unique speed called "The Speed Of Light."
> >
> >
> > There is only one speed for the speed of light, c.  Light does NOT
> > travel more slowly in a medium!!! This is because light ONLY travels in
> > a vacuum (between the atoms!).  Please refer to the Feynman Lectures.
>
>
> So, is it wrong to teach K-6 students that light slows down while it's
> inside a transparent material?   If so, then I don't know how to explain
> lenses and prisms to eleven year old children.
>
> Is it wrong to teach that light is made of propagating EM fields?  If so,
> if light is "really" Feynman virtual photons, how can I explain certain
> field phenomena such as polarization?  Also how do I explain magnets?
>
> I seek suggestions for ways to improve the above passage from my
> "Misconceptions" page.  (or any other parts of that page.)