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Re: [Phys-L] Why is light slower in glass?


On 2016, Jan 24, , at 15:46, John Denker <jsd@av8n.com> wrote:

On 01/24/2016 01:04 PM, Savinainen Antti wrote:

I was asked the question in the title some time ago.

My answer:
a) To a first approximation, the molecules in the glass are
little oscillators. Masses on springs.

The Drude-Lorentz theory

An incomplete theory is given by John Strong in Concepts of Classical Optics.

https://www.aip.org/history-programs/niels-bohr-library/oral-histories/28279


e) When you drive something at a frequency far below resonance,
the deflection will be nearly in phase with the force (ignore
the mass, and you're left with Hooke's law). This stands in
contrast to the far-above-resonance case, where the deflection
is 180° out of phase with the force (ignore the spring, and
you're left with Newton's second law).

which explains anomalous dispersion.


At X-band the oscillators are disks or wires. To show anomalous dispersion one uses wires of length to match the wavelength. An array of them below (freq.), for example a prism, exhibits “normal” refraction; above reversed.

I have an HP broad band generator (Gun diode?), which I pray works, to test this wire “meta material”

Idea(s) from Kock and Strong.

"Sound Waves and Light Waves"


https://en.wikipedia.org/wiki/Winston_E._Kock


I have demonstrated a lens made of thumbtacks to show ordinary refraction, And a great a demo of rotation of the plane using Cu springs. I don’t think there’s a natural chiral material to do this. Finally there is a natural material with which to make a 1/4 wave plate, it’s wood. I’ve also used the unnatural material, plastic straws for this, also.



For the next level of detail:
Feynman volume I chapter 31: "The Origin of the Refractive Index"
http://www.feynmanlectures.caltech.edu/I_31.html

Also:
There should be a way to simulate this in a ripple tank, using
some lightweight floating objects to simulate the oscillators
that are excited by the incoming wave and then re-radiate ...
but I have never tried it and there are a lot of things that
could go wrong, so this is just a conjecture. It's not an
entirely faithful model. Has anybody done this?


Assignment for a science faire project!

I suspect the refractor be an array of mini rubber balloons; would, unfortunately, be rather dissipative. There are v. high Q rubbers, but I suspect of very low compliance. What gas has low viscosity? Sulfur hexafluoride cooled? Gaseous alkanes?

Backwards! lowest on the table I found is H2, and only about half many others (including air) ~ 8 micro Pascal-second

bc has firm grasp of kinetic theory. :-)

This reminds me of reverse osmosis of H2 thru a metal membrane (hot Pd, IIRC). Bad memory: what I used as a non-reactive thermal bath for my disst.


I realize that the essence of critical thinking is to reconcile
each new thing you learn (e.g. waves) with things you've heard
previously (e.g. photons) ... but sometimes this comes down to
realizing that most of what you've been told about photons is
just wrong. As a matter of engineering, nobody in his right
mind would measure the refractive index using photon counters.
The experiments that actually get done are well described in
terms of waves.

The student suggested a YouTube video by prof.
Merrifield: <https://www.youtube.com/watch?v=CiHN0ZWE5bk>

Here is another related video on refraction by prof. Moriarty:
<https://www.youtube.com/watch?v=YW8KuMtVpug&feature=youtu.be>

In terms of the physics, both of those videos say a lot of things
that are correct, and avoid making any serious technical mistakes.

However, in terms of the pedagogy, it's a disaster. You should
start by telling students the right answer. At the introductory
level, students do not need to hear shaggy-dog stories about all
the possible wrong answers. Later, *after* they have a decent
grasp of the right answer, there will be time to spiral back and
understand what's wrong with each of the wrong ways of looking
at things.

Both of those videos blither at length about a whole bunch of
irrelevant stuff. I'm not even going list all the irrelevant
stuff. It's not even worth explaining why it's irrelevant.


On 01/24/2016 01:47 PM, Shahram Mostarshed replied:

Here'sa good explanation by Paul Hewitt:

https://www.youtube.com/watch?v=6fSf0lfRLKY

As usual, Hewitt gets the wrong answer. There are a couple
of ways of understanding why it cannot possibly be correct,
as discussed in the other two videos.
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bc a fan of John Strong, and should read more of Feynman et alii.

I have posted more cent. old 3D scanned slides.

http://www.cleyet.org/glass_images/