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X-rays in Crystals: Newsletter Challenge (05/03/11)

Posted May 01, 2011 5:01 PM

This month's Challenge Question:

Most electromagnetic waves exhibit an index of refraction >1 in most media, so when the wave enters a medium, its speed is less than the speed of light in a vacuum. If "n" is the index of refraction and "c" is the speed of light in a vacuum, the speed of the electromagnetic wave in a particular medium is given by v=c/n. However, this is not the case when X-rays penetrate crystals. In this case, the index of refraction is <1. How can you explain this? Is it possible that X-rays in crystals travel at a speed higher than the speed of light in a vacuum?

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#1

Re: X-rays in Crystals: Newsletter Challenge (05/03/11)

05/01/2011 11:59 PM

Ideally, Newsletter Challenges should not rely on little-known principles for their resolution; rather, the challenge should be to explain some question by relying on the ingenious usage of more basic principles (such as algebra or geometry versus calculus or functional analysis).

This instance may be an analogy with a sonic boom in air, or Cerenkov radiation in water or glass or whatever. I don't know yet that it entails travel at faster than light in vacuum.

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#3
In reply to #1

Re: X-rays in Crystals: Newsletter Challenge (05/03/11)

05/02/2011 11:07 AM

You're welcome to submit questions.

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#2

Re: X-rays in Crystals: Newsletter Challenge (05/03/11)

05/02/2011 10:23 AM

I'm not sure that the index of refraction is what we should be talking about. I think the right term is defraction and not refraction. X-rays have wave lengths small enough that they interact with individual atoms and reflect off sort of like surface waves reflecting off of a cylinder. Based on the spacing of the atoms and the wave length of the x-rays, these cylindrical waves interact to give something looking like a straight wave.

Thanks,

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#4

Re: X-rays in Crystals: Newsletter Challenge (05/03/11)

05/02/2011 11:42 AM

Actually the v you get from this function (v=c/n) is the phase speed of the wave, it is not a real speed, it's an artifact used on calculations. So it could be as 'faster' than light your heart desires. S.M.

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#5

Re: X-rays in Crystals: Newsletter Challenge (05/03/11)

05/02/2011 10:27 PM

I suspect that x-rays are a lot smaller than the atoms and bounce off the atoms. Bragg's law, crystallography, etc This I suspect is called Bragg Scattering

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#6

Re: X-rays in Crystals: Newsletter Challenge (05/03/11)

05/02/2011 11:45 PM

To start with, electromagnetic waves do not exhibit an index of refraction. Index of Refraction is a property of a medium.

However, as another poster has pointed out, this phenomenon is in fact "phase velocity", and is generally an X-ray phenomenon, and occurs on a micro scale only. Wikipedia has an excellent article: http://en.wikipedia.org/wiki/Refractive_index

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#7
In reply to #6

Re: X-rays in Crystals: Newsletter Challenge (05/03/11)

05/03/2011 10:13 AM

There are two different speeds confused ever since physics is taught.

The first is the physical signal speed. It is the speed of light for any photons, meaning the speed of light in the given medium: c/n. It carries energy, therefore information.

The other speed I better describe: take a very strong laser pointer, and start painting the clouds with it. When you move your wrist fast enough, your spot of light moves ACCROSS faster than the speed of light. You can see it, calculate it, measure it. Is it real? Yes, but. You asked the wrong question. Your pointer emits individual photons. One impinges here, the other there. There is no connection between the individual photons whatsoever. A speed defined this way is real, but carries no energy, therefore no information. The individual photons do. And yes, Texas Instruments built and sold an oscilloscope, where the electron beam swept across the screen at such speeds. It did not bother me at all.

I have a question back to the author of the original question. And it is real. If you think real microscopic when a photon approaches matter (glass, metal, whatever), how fast that photon have to "know" that it is moving in a different refractive index matter? The answer is obvious on the large scale, but it is not obvious on that small scale.

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#10
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Re: X-rays in Crystals: Newsletter Challenge (05/03/11)

05/04/2011 12:32 AM

is the electromagnetic wave split into their components?

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#8

Re: X-rays in Crystals: Newsletter Challenge (05/03/11)

05/03/2011 1:13 PM

The group velocity (signal speed) is always less than the speed of light. For massive particles the phase velocity is always greater and satisfies the relation vg*vph=c**2. The so-called negative index materials (metamaterials) have epsilon<0 and mu<0 so that they refract contrary to how we usually expect. This can give "superlensing" and can be observed with thin sheets of silver (as well as the fancy microwave domain metamaterials). In the case of x-rays in metals I think this is in the anomalous dispersion regime so that there is strong damping at work. There was some famous confusion about this in the 1990's where people seemed to get gaussian pulses to travel faster than light. This was true for the central peak of the packet but the packet was so attenuated that it was deceptive. The leading edge of the packet was still subluminal.

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#9
In reply to #8

Re: X-rays in Crystals: Newsletter Challenge (05/03/11)

05/03/2011 10:15 PM

Nnno, definitely not . Quoting definitions explains not a blessed thing. Particles with mass always are slower than c (as the speed of light). Photons are always at c. Not lower, not above, in the matter they travel.

Metamaterials show negative epsilon or mu in a limited frequency band, yes. They do not negate physics, as we knew at all. Quite interesting area, but drawing any conlusions because math allows is, just wild and not justified. Plenty of people forget, that the physical phenomenon IS THE REAL THING, while math description is just a reflection. It can be manipulated right or wrong.

Research on the bleeding edge is full of false positives (and negatives). Primacy demands of publication and egos put a tremendous pressure on early publication, even at the risk of mistake. Cchaf describes prime examples of it.

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#12
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Re: X-rays in Crystals: Newsletter Challenge (05/03/11)

05/04/2011 7:27 PM

C is usually defined as the speed of light in a vacuum. Light slows down when it enters a media. So, no, light does not always travel at c.

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#19
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Re: X-rays in Crystals: Newsletter Challenge (05/03/11)

05/31/2011 10:18 AM

No, the light ALWAYS travels at c, even inside the materials. It is just the "apparent" velocity of light that seems to be lower than c. And that's because the absorption and retransmission of photons by the atoms of the material provides a kind of delay.

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#11

Re: X-rays in Crystals: Newsletter Challenge (05/03/11)

05/04/2011 5:04 AM
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#13

Re: X-rays in Crystals: Newsletter Challenge (05/03/11)

05/06/2011 2:15 PM

The ray of light impinges upon the chrystal and subsequently emerges as a ray of light from the opposite face, having made inumerous collisions with the atoms in the chrystal. Why isn't the ray scattered into all directions, so that only a small fraction exits in the original direction????

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#14

Re: X-rays in Crystals: Newsletter Challenge (05/03/11)

05/07/2011 11:16 AM

Older&Wiser, This is the oft forgotten Ewald-Osseen extinction theorem of 1915. This was a big time in physics after Einstein's revolution so some important results didn't get into the text books. This is in Born and Wolf's Principles of Optics. It is an amazing bit of cancellation to create the transmitted and reflected beams from all these scatterers.

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#15

Re: X-rays in Crystals: Newsletter Challenge (05/03/11)

05/10/2011 12:12 PM

Most likely the keyword is "electromagnetic."

In this case the electromagnetic properties of the x-ray wave will couple with the crystal on the surface of the crystal.

Pure crystal structures are balanced electrically & magnetically.

When light enters a crystal it is broken up into its electrical and magnetic properties or waves,momentarily (almost instantly) and recombined upon exit if you will... Giving the illusion that the electromagnetic wave has traveled faster through the medium.

During transmission the crystal vibrates creating harmonic frequencies. The electric and magnetic properties of the transmitting wave is combined with the harmonic frequencies. All of the above wave elements are concentrated and potentially accelerated by the atomic arrangement within the crystal's bonds when multiple harmonics are present.

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#16

Re: X-rays in Crystals: Newsletter Challenge (05/03/11)

05/24/2011 1:25 PM

Is it possible that X-rays in crystals travel at a speed higher than the speed of light in a vacuum?

Yes. (beyond the expression "Anything's possible," however hard to structurally imagine or realize.) The sense of the situation points to the harmonic pulsing energies a 'purified' crystal lattice structure produces. As a layman/artist, the example that comes immediately to mind is the quartz crystals that once-upon-a-time powered watches and regulated their works with harvestable 'pulsar' energy.

The logic at work in the postulate (refraction <1) is that the x-ray somehow feeds off the crystal's pulsing structural energy to accelerate through the medium, which to the x-ray acts as a microcosmic linear accelerator.

Just my thought.

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#17

Re: X-rays in Crystals: Newsletter Challenge (05/03/11)

05/24/2011 4:08 PM

Diffraction slows the travel of x-rays through a crystal by diverting the ray path through the crystalline lattice and thus changing the path length through the chrystal. In other words the travel distance is increased, but the speed of the electromagnetic radiation remains unchanged. Each unique crystalline structure diffracts x-rays differently. Diffraction patterns can be measured and used for identification of naturally occuring minerals and man-made crystalline compounds.

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#18

Re: X-rays in Crystals: Newsletter Challenge (05/03/11)

05/24/2011 9:14 PM

It seems to me this question confuses 'apples with oranges'. There is a quite a bit in astronomy that suggests X-rays are faster than light.

This crystal refraction concept might actually be a way of finding out by how much, (if in fact they slowed in a crystal)

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#20
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Re: X-rays in Crystals: Newsletter Challenge (05/03/11)

05/31/2011 10:52 AM

This sort of thing keeps coming up because of popular science articles are being written by sensationalists or people that don't understand the subtleties involved. It ends up confusing many people, even those who are engineers and scientists. The disturbance can travel faster than light eg url="http://www.msnbc.msn.com/id/34938302/ns/technology_and_science-space/t/cosmic-currents-may-move-faster-light/"] but this is deceptive. Imaging shining your laser pointer at the face of the moon and swinging it across it. The moon has about 1/2 a degree in the sky. It is over 3000 km across. This means that if you swing your pointer at faster than 43 deg/sec the spot moves across it at faster than the speed of light but no photon or particle is moving faster than c.

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#21

Re: X-rays in Crystals: Newsletter Challenge (05/03/11)

06/08/2011 10:13 AM

Index of refraction historically was introduced from optics that deals with photon interaction on the external electron shells. In opposit, gamma rays have absolutely different laws and main processes are photoeffect, compton and pair productions. X- rays belong to intermediate region where optics laws almost stop works due to the fact that for X- rays we have additional interaction photons with internal K, L and other shells. In this case people often use Snell's law for index of refraction with only purpose to get total external angle of refraction. All other calculations it is better to use special software such as Geant, MCNP and so on. It is impossible to use optics laws for infrared photons as well.

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#22
In reply to #21

Re: X-rays in Crystals: Newsletter Challenge (05/03/11)

06/08/2011 11:47 AM

Does this mean it makes no sense to define a complex dielectric constant here? If the medium is thin enough I am pretty sure this is true but I don't know in general.

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#23

Re: X-rays in Crystals: Newsletter Challenge (05/03/11)

07/06/2011 9:26 PM

When light is refracted the light is obelisk in aperture involvement this allows the seer to establish the vector in triplicate and the aperture involvement is lost in conjecture when applied to your crystal the oblisk is only double .4times the indentured divided by the apature will comferm this. It is possible that the speed of light is achevied in congluent yeild .Lets see this is challenging dipoild multiplyed by circcumphrence divided by the apature indivisable by nature anser YES.ZEN M.L.D.

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