| Topic: |
Science > Physics |
| User: |
"Tony" |
| Date: |
08 Mar 2005 01:27:10 AM |
| Object: |
graded index of refraction |
I'd like to get this right. If you have a material that has a graded
index of refraction in the direction of propogation over the wavelength
range of interest, say going from 1 to 1.5 over a distance of many
wavelengths of light, will there be a reflection?
My particular focus (pun not intended) is for transition zones that are
pretty deep, but a good question would have been how shallow a
transition zone does it take before the old (n0-n1)^2/(n0+n1)^2
approximation fails?
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| User: "CWatters" |
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| Title: Re: graded index of refraction |
08 Mar 2005 02:39:29 AM |
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"Tony" <ajw27703@yahoo.com> wrote in message
news:1110266830.169929.47400@o13g2000cwo.googlegroups.com...
I'd like to get this right. If you have a material that has a graded
index of refraction in the direction of propogation over the wavelength
range of interest, say going from 1 to 1.5 over a distance of many
wavelengths of light, will there be a reflection?
Isn't that how a desert mirage is created?
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| User: "Nosterill" |
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| Title: Re: graded index of refraction |
08 Mar 2005 04:07:38 AM |
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CWatters wrote:
"Tony" <ajw27703@yahoo.com> wrote in message
news:1110266830.169929.47400@o13g2000cwo.googlegroups.com...
I'd like to get this right. If you have a material that has a
graded
index of refraction in the direction of propogation over the
wavelength
range of interest, say going from 1 to 1.5 over a distance of many
wavelengths of light, will there be a reflection?
Isn't that how a desert mirage is created?
In a mirage, the refractive index is graded in a direction almost
perpendicular to the direction of propogation.
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| User: "CWatters" |
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| Title: Re: graded index of refraction |
09 Mar 2005 01:04:47 AM |
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"Nosterill" <robin@davinoptronics.com> wrote in message
news:1110276458.927487.132090@z14g2000cwz.googlegroups.com...
CWatters wrote:
"Tony" <ajw27703@yahoo.com> wrote in message
news:1110266830.169929.47400@o13g2000cwo.googlegroups.com...
I'd like to get this right. If you have a material that has a
graded
index of refraction in the direction of propogation over the
wavelength
range of interest, say going from 1 to 1.5 over a distance of many
wavelengths of light, will there be a reflection?
Isn't that how a desert mirage is created?
In a mirage, the refractive index is graded in a direction almost
perpendicular to the direction of propogation.
Ah yes I didn't spot that twist in the original.
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| User: "Nosterill" |
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| Title: Re: graded index of refraction |
08 Mar 2005 04:18:54 AM |
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Are you considering this as an anti-reflection process? It has been
looked at before and theoretically very high performance graded index
AR coatings could be designed. The problem is that the lowest
refractive index materials available are much closer to 1.5 than to 1,
so not much could be achieved. There was some work done in Sweden in
the sixties producing graded AR's for germanium, where a reduction from
index 4 to 1.35 was worth doing.
The other common case of refractive index graded in the direction of
propogation is in the Gradium materials, but these are intended to give
extended correction possibilities with spherical surfaces and do
nothing unusual for reflection.
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| User: "Tony" |
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| Title: Re: graded index of refraction |
08 Mar 2005 07:18:58 AM |
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I wasn't thinking of A/R's specifically but of course that would be a
practical application. MgF starts at 1.3 or so, and I expect if it goes
down porously the surface may be optically "indistinct" for lack of a
better word. My concern has to do with having some technically austite
people shoot holes in a piece of fiction I'm writing that could use the
notion of a graded interface.
Extending the notion, I thought the reason thunder 'reflects' off
clouds is because sound velocity is probably a bt different in visible
moisture, but sound is a compressive wave,not a transverse one.
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| User: "tadchem" |
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| Title: Re: graded index of refraction |
08 Mar 2005 08:32:10 AM |
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Tony wrote:
I wasn't thinking of A/R's specifically but of course that would be a
practical application. MgF starts at 1.3 or so, and I expect if it
goes
down porously the surface may be optically "indistinct" for lack of a
better word.
Aerogels (Google this term) are structured materials made of solids and
gas/empty space. The densities and refractive indices are less than
those for the pure solid.
They can be transparent but typically are opalescent/milky when pore
size is not strictly controlled.
<http://images.google.com/images?q=aerogels&hl=en&lr=&sa=N&as_qdr=all&tab=wi>
A series of progressively denser aerogels can accomplish the graduation
for n = 1.0 to n = (whatever) in steps.
My concern has to do with having some technically austite
people shoot holes in a piece of fiction I'm writing that could use
the
notion of a graded interface.
Extending the notion, I thought the reason thunder 'reflects' off
clouds is because sound velocity is probably a bt different in
visible
moisture, but sound is a compressive wave,not a transverse one.
I have never heard of thunder 'reflecting' but I am very familiar with
sounds *refracting* through air of variable density. Sound carries
very well when the ground temperature is lower than the air temperature
(across a calm lake), and very poorly when the ground temperature is
higher than the air temperature (in the desert). It is refracted the
same way light is refracted in desert 'lake' mirages or the sailor's
Fata Morgana.
Basically, light and sound tend to bend away from 'thinner' media
(thinner acoustically means faster speed of sound; thinner optically
means lower refractive index).
Tom Davidson
Richmond, VA
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| User: "Tony" |
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| Title: Re: graded index of refraction |
08 Mar 2005 09:23:36 AM |
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Tom, I don't want to get too far away from my initial question, but I
know compressive waves will 'reflect' from interfaces of different
density, and have a vague suspicion they may do so also from abrupt
interfaces (abrupt meaning much less the sound's wavelength) between
media with different propogation velocities. I'd further guess sound is
faster in clouds.
While aloft at something like 10,000 feet I've been (too) close to
lightening flashes, heard the first report and others not much later.
There was sure not enough time for the second sounds to have gotten to
the ground and back. So, one observable flash, a boom in less than two
seconds, other booms soon after that: they were loud enough to be heard
through a good headset and over the noise of an IO360 pulling my
airplane. I'm pretty sure I was hearing secondary (reflected) energy.
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| User: "bz" |
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| Title: Re: graded index of refraction |
08 Mar 2005 10:16:46 AM |
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"Tony" <ajw27703@yahoo.com> wrote in news:1110295416.498759.199420
@o13g2000cwo.googlegroups.com:
Tom, I don't want to get too far away from my initial question, but I
know compressive waves will 'reflect' from interfaces of different
density, and have a vague suspicion they may do so also from abrupt
interfaces (abrupt meaning much less the sound's wavelength) between
media with different propogation velocities. I'd further guess sound is
faster in clouds.
While aloft at something like 10,000 feet I've been (too) close to
lightening flashes, heard the first report and others not much later.
There was sure not enough time for the second sounds to have gotten to
the ground and back. So, one observable flash, a boom in less than two
seconds, other booms soon after that: they were loud enough to be heard
through a good headset and over the noise of an IO360 pulling my
airplane. I'm pretty sure I was hearing secondary (reflected) energy.
Although I agree with you about sound being reflected from discontinuities,
I can propose another mechanism for some of those reports.
The 'path' to ground may have segments that are perpendicular to you. Each
such segment could produce a separate shock wave for you to hear.
A rather uniform bolt, with the path going away from you, on the otherhand,
should produce a long, sustained, report.
Now, I am going to argue against myself,[perhaps only a little].
I understand (have never heard with my ears) that distant cannon fire can
sound like thunder accompanied by a sustained report. Since the cannon fire
represents a 'point source', the rolling sound must be produced by
reflections and or refraction.
I guess that isn't really against my explanation for the multiple bangs you
heard.
--
bz
please pardon my infinite ignorance, the set-of-things-I-do-not-know is an
infinite set.
bz+sp@ch100-5.chem.lsu.edu
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| User: "" |
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| Title: Re: graded index of refraction |
08 Mar 2005 04:27:54 PM |
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In article <1110308885.572328.29730@z14g2000cwz.googlegroups.com>, "Tony" <ajw27703@yahoo.com> writes:
Thanks for the comment re lightening, all, they make sense to me.
Any thoughts on the initial Q re graded refractive indexes?
This is actually used, in x-ray surface scattering. With grading
present, the "step change" reflectivity is modified, being multiplied
by a factor proportional to the square of the Fourier transform of n
(n is a function of the coordinate normal to the surface). So, you
can get enhanced reflectivity at some wavelenghts or angles of
incidence, diminished at other.
Mati Meron | "When you argue with a fool,
meron@cars.uchicago.edu | chances are he is doing just the same"
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| User: "" |
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| Title: Re: graded index of refraction |
08 Mar 2005 06:20:37 PM |
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In article <1110326685.931333.62770@l41g2000cwc.googlegroups.com>, "Tony" <ajw27703@yahoo.com> writes:
Thanks, Mati, but I think in the circumstances you're describing the
wavelengths of the x rays are the same order of magnitudena as the
surface changes.
Well, I would assume the thickness of the graded layer, for the
optical case, to be much larger than in the x-ray case, as well. "tis
all really a matter of impedance matching.
I'm fairly well convienced that for my purposes ( it's a novel,not a
peer reviewed paper, for god's sake) I'm going to claim no reflections
from such a surface.
Won't you find it more interesting to have a surface which doesn't
reflect in one direction but does reflect in another? Can be some
plot possibilities in this:-)
Thanks.
You're welcome.
Mati Meron | "When you argue with a fool,
meron@cars.uchicago.edu | chances are he is doing just the same"
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| User: "" |
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| Title: Re: graded index of refraction |
08 Mar 2005 11:53:58 PM |
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In article <1110334602.190854.37570@f14g2000cwb.googlegroups.com>, "Tony" <ajw27703@yahoo.com> writes:
Not reflect in one direction but does in the other is trivial --any
mirror with a painted back side.
No, I don't mean such extreme. Rather something like "not reflecting
when viewed straight on, but reflecting when (same surface) is viewed
at, say, 45 degrees.
Now one way transmission is another matter, isn't it?
Aye, quite another. As a rule, what goes one way, can go the other.
Mati Meron | "When you argue with a fool,
meron@cars.uchicago.edu | chances are he is doing just the same"
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| User: "Tony" |
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| Title: Re: graded index of refraction |
09 Mar 2005 02:25:29 AM |
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I can give you 50% reflection at 45 degrees, zero at normal, (usual
assumptions, collimated beam) can't see an obvious way to do better
than that.
Won't help in the book, though.
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| User: "" |
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| Title: Re: graded index of refraction |
09 Mar 2005 03:08:08 AM |
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In article <1110355305.133715.237490@g14g2000cwa.googlegroups.com>, "Nosterill" <robin@davinoptronics.com> writes:
Large changes of reflectivity with angle are quite routine, for
reasonably narrow bandwidths. I have often used this for hollow Schmidt
"prisms". They tend to get called anglechroics.
In the x-ray domain you'll get large changes using multilayers. Which
are based, really, on a graded (periodically) index of refraction.
Mati Meron | "When you argue with a fool,
meron@cars.uchicago.edu | chances are he is doing just the same"
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| User: "" |
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| Title: Re: graded index of refraction |
09 Mar 2005 03:12:19 AM |
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In article <1110356729.207971.177870@z14g2000cwz.googlegroups.com>, "Tony" <ajw27703@yahoo.com> writes:
I can give you 50% reflection at 45 degrees, zero at normal, (usual
assumptions, collimated beam) can't see an obvious way to do better
than that.
With periodically graded index of refraction you can do much better
than this.
Won't help in the book, though.
Oh, well. Don't think I can help more.
Mati Meron | "When you argue with a fool,
meron@cars.uchicago.edu | chances are he is doing just the same"
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| User: "Nosterill" |
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| Title: Re: graded index of refraction |
09 Mar 2005 02:01:45 AM |
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Large changes of reflectivity with angle are quite routine, for
reasonably narrow bandwidths. I have often used this for hollow Schmidt
"prisms". They tend to get called anglechroics.
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| User: "Tony" |
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| Title: Re: graded index of refraction |
08 Mar 2005 08:16:42 PM |
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Not reflect in one direction but does in the other is trivial --any
mirror with a painted back side. Now one way transmission is another
matter, isn't it?
I'm trying to stay on the legal side of the laws of thermodynamics.
Ohm's suggestion or Newton's Guestimates of Motion are a different
matter.
..
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| User: "Tony" |
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| Title: Re: graded index of refraction |
08 Mar 2005 06:04:45 PM |
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Thanks, Mati, but I think in the circumstances you're describing the
wavelengths of the x rays are the same order of magnitudena as the
surface changes.
I'm fairly well convienced that for my purposes ( it's a novel,not a
peer reviewed paper, for god's sake) I'm going to claim no reflections
from such a surface.
Thanks.
BTW, the sounds from lightening strokes propogating sure fit my
observations better than do my thought that I was hearing reflections
aka echos.
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| User: "Franz Heymann" |
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| Title: Re: graded index of refraction |
09 Mar 2005 02:50:52 PM |
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"Tony" <ajw27703@yahoo.com> wrote in message
news:1110326685.931333.62770@l41g2000cwc.googlegroups.com...
Thanks, Mati, but I think in the circumstances you're describing the
wavelengths of the x rays are the same order of magnitudena as the
surface changes.
I'm fairly well convienced that for my purposes ( it's a novel,not a
peer reviewed paper, for god's sake) I'm going to claim no
reflections
from such a surface.
Thanks.
BTW, the sounds from lightening strokes propogating sure fit my
observations better than do my thought that I was hearing
reflections
aka echos.
If you are writing a novel, you might consider using the correct
spelling for "lightning"
{:-((
--
Franz
"A first-rate laboratory is one in which mediocre scientists can
produce outstanding work"
P.M.S. Blackett
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| User: "Tony" |
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| Title: Re: graded index of refraction |
09 Mar 2005 04:36:39 PM |
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Good point. Goes to show the need for editors.
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| User: "Tony" |
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| Title: Re: graded index of refraction |
08 Mar 2005 01:08:05 PM |
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Thanks for the comment re lightening, all, they make sense to me.
Any thoughts on the initial Q re graded refractive indexes?
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| User: "tadchem" |
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| Title: Re: graded index of refraction |
08 Mar 2005 01:57:40 PM |
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Back to the aerogels I mentioned in my earlier post.
Google the term
<http://www.google.com/advanced_search?hl=en>
and look at some of the *images* of aerogels. One of the more
interesting properties of their appearance is that it is often very
hard to see the surface of these materials. That is because the
surfaces don't reflect light very well.
If you graded them in successive layers from low density to high
density, with the high density in contact with the solid material, you
would have optics that did not reflect light, and you would have a
gradually increasing refractive index.
With a little work you might even be able to approach nearly continuous
gradations of refractive index in the direction of propagation.
Tom Davidson
Richmond, VA
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| User: "tadchem" |
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| Title: Re: graded index of refraction |
08 Mar 2005 10:06:22 AM |
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Tony wrote:
Tom, I don't want to get too far away from my initial question, but I
know compressive waves will 'reflect' from interfaces of different
density, and have a vague suspicion they may do so also from abrupt
interfaces (abrupt meaning much less the sound's wavelength) between
media with different propogation velocities. I'd further guess sound
is
faster in clouds.
You are correct. Sound reflections are called 'echoes' and they can be
heard from barriers as close as a few meters or as far as hundreds of
meters.
While aloft at something like 10,000 feet I've been (too) close to
lightening flashes, heard the first report and others not much later.
There was sure not enough time for the second sounds to have gotten
to
the ground and back. So, one observable flash, a boom in less than
two
seconds, other booms soon after that: they were loud enough to be
heard
through a good headset and over the noise of an IO360 pulling my
airplane. I'm pretty sure I was hearing secondary (reflected) energy.
Lightning is typically described as made up of several path segments
(called mesotortuous segments). Each one is a separate source of and
explosion of air producing a distinct 'report' of thunder. The sounds
from many of these segments combine to produce the unique sound of
thunder, which analysis shows sounds distinctly different from
different locations. Lightning can be several miles long, and the sound
from each segment travels a different path to any given observer, and
has its own arrival time. Computer analysis of thunder can be used to
reconstruct the shape of the entire lightning 'bolt.'
<http://www.islandnet.com/~see/weather/elements/thunder2.htm>
Tom Davidson
Richmond, VA
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