# Thread: the speed of light through solids

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## the speed of light through solids

I have seen the data on speed through solids, as it is dense the particles line up when it is struck at one end the distortion travelled instantly through. like having a tube of cocktail sticks and pushing one, the result is instant as it comes out the other end as far as you have pushed it. the tests results on speed through the solid came out as faster than light over the short metal test subject as the passage through was instant, any views on this.

2. Originally Posted by blackmansdream
... any views on this.
Perhaps you were thinking of the speed of sound. I've never seen anything about light going faster in a solid...

3. Speed of light in a solid would be c divided by the index of refraction. Since the index of refraction is always greater than one (except with metamaterials, where it may be negative, the meaning of which has me completely confuzzled), the speed of light in a solid will always be less than c.

Speed of sound in a solid is another issue: it's proportional to the solid's modulus of elasticity divided by the density. iirc, diamond has the highest speed of sound of any solid likely to be hanging around on Earth.

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The speed of sound in a material results from the electrons
of a pushed atom shoving against the electrons in the next
atom. The faster the electrons spring back, the faster the
shove is transmitted through the material.

The speed of light in a material results from the light being
absorbed and then re-emitted by the electrons in the atoms.
The more often a photon is absorbed, and the longer the
delay before it is re-emitted, the slower the light moves
through the material. In transparent materials, the light
is re-emitted very, very quickly.

-- Jeff, in Minneapolis

5. This sounds very like the "infinitely dense material". The idea is that if you could get a long stick, then moving one end means the other end moves, and you can use this to transmit faster than light speed. Nice idea, but fails since light will reach the end of the stick before the movement (unless the material is "infinitely dense", which is a bit difficult to achieve).

Alternatively, you may have heard about the experiments where light is fired across a fixed length, but encounters an obstacle - say something reflecting back 90%. The remaining 10% are found to arrive faster than their unblocked counterparts. Does this mean faster than c?
No. IIRC, since light is travelling as a wave, it is effectively the leading edge of the 10% wave that gets picked up (something like that anyway! Does anybody have any idea what I'm talking about referring to? Could be the Hartman effect) Still doesn't allow transmission at faster than c, though

6. Originally Posted by blackmansdream
I have seen the data on speed through solids, as it is dense the particles line up when it is struck at one end the distortion travelled instantly through. like having a tube of cocktail sticks and pushing one, the result is instant as it comes out the other end as far as you have pushed it. the tests results on speed through the solid came out as faster than light over the short metal test subject as the passage through was instant, any views on this.
Where is this data you've seen? First guess is that you might be mistaken on what you've read or you've read is not from a reputable source.

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Rob,

Rather than infinite density, your stick requires perfect
stiffness, or incompressibility. Density by itself does not
determine the speed at which force is transmitted through
material. As swampyankee said, the speed of sound in
diamond is extremely high, but diamond is not especially
dense. The speed of sound is high because the bonds
between atoms are short and strong-- so a change in
position of one atom causes movement of an adjacent
atom very quickly. The maximum speed that the force
can be transmitted from one atom to another is the
speed of light, since it actually is light which transmits
the electric force between electrically-charged particles.
It takes time for the particles to be accelerated, so the
force is transmitted through the bulk material at a much
slower speed. But the lighter (less dense) the particles,
the more rapidly they can be accelerated. So there is
actually a speed advantage to being *less* dense.

-- Jeff, in Minneapolis

8. ...the speed of light through solids...

AFAIK, light doesn't typically travel through solids. It gets reflected or scattered off solids.

9. Originally Posted by Cougar
...the speed of light through solids...

AFAIK, light doesn't typically travel through solids. It gets reflected or scattered off solids.

10. Originally Posted by AndreH
There are a lot more solids that are opaque to light than there are solids that are semi-transparent to light. That's why I said "typically."

11. Originally Posted by Cougar
There are a lot more solids that are opaque to light than there are solids that are semi-transparent to light. That's why I said "typically."
That is maybe true, even there is a big bunch of metallic oxides and nitrides which are (semi)-transparent for light. I was confused by your confused smily. Why is it confusing to speak of the speed of light in solids if you know there are transparent ones? And don't take this too serios, I should have posted a smily. I myself hate it to get nit picked for non precise short hand answeres.

12. Originally Posted by swampyankee
Speed of light in a solid would be c divided by the index of refraction. Since the index of refraction is always greater than one (except with metamaterials, where it may be negative, the meaning of which has me completely confuzzled), the speed of light in a solid will always be less than c.
"common misconception" according to http://en.wikipedia.org/wiki/Refract..._index_below_1

Originally Posted by Jeff Root
The speed of light in a material results from the light being
absorbed and then re-emitted by the electrons in the atoms.
The more often a photon is absorbed, and the longer the
delay before it is re-emitted, the slower the light moves
through the material. In transparent materials, the light
is re-emitted very, very quickly.
I think we've had this discussion before? I think that is an over-simplified model. If the photons were actually absorbed, the material probably wouldn't be transparent. The EM interaction that occurs is at the wave-level, not the individual particle-level.

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The description I chose to give is a particle description
rather than a wave description. Both descriptions should
work, but they cover different aspects of the behavior of
light, and they work in such different ways that it is famously
hard to fathom that they both describe the same thing.

If light is indeed composed of particles, then those particles
*must* interact with charged particles, and it would be silly
to claim that I can't describe such interactions. As far as I
know, the description of individual photons interacting with
individual charged particles works, but it doesn't cover
everything. It can't, for example, predict how a beam of
light will be refracted when it passes from one transparent
medium to another. That requires a wave description.

I agree that my description is simplified. I'm not sure it is
over-simplified. My understanding and interpretation is that
if the absorption and re-emission is *very* rapid-- that is,
nearly instantaneous-- the re-emitted photon can move in
practically the same direction as the absorbed photon.

That particle description was intended to explain the delay
light experiences when travelling through a material. It is
not useful for explaining everything that light does.

There may be a technical definition of "photon absorption"
which I do not know and am therefore misusing. I have
taken the assertions I've seen that when a photon interacts
with a charged particle, it is absorbed. If it is not absorbed,
there is no interaction. It is all-or-nothing.

-- Jeff, in Minneapolis

14. Originally Posted by Jeff Root
If light is indeed composed of particles, then those particles
*must* interact with charged particles, and it would be silly
to claim that I can't describe such interactions.
How do those particles interact with themselves, in the double slit experiment? Surely there is no absorbing going on right?

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From the lack of response to that very salient question, I'm
beginning to suspect that Jeff is completely at sea. He might
have been hoping for some kind of deus ex reticula to save
him. Waves, waves everywhere, nor any molecule to drink...

-- What? Who, me?

16. Originally Posted by grapes
I think we've had this discussion before? I think that is an over-simplified model. If the photons were actually absorbed, the material probably wouldn't be transparent. The EM interaction that occurs is at the wave-level, not the individual particle-level.
You can make an analogy with electrical circuits with capacitors or inductors that might be useful, depending on your familiarity with filters/signal analysis. RC/LC/RLC circuits can produce a phase shift on an input waveform, and can also be described in extremely simplified terms as the capacitor charging and then discharging, or an inductor storing energy in its field and then releasing it. The leading edge of the waveform is not delayed, though, but will be changed in shape (or from another perspective, frequency content/relative phasing of different frequencies). Light passing through a material (solid, liquid, or gas) basically encounters an RLC system with every charged particle in its path.

You have charged particles with mass and under the influence of neighboring charged particles being accelerated by a changing electromagnetic field and producing electromagnetic radiation themselves in response. I think the quantum mechanical picture is essentially the same, though with particle behavior cropping up at times.

As for solids letting light through, there's a great many solids that are transparent in visible light or in other bands, and a great many liquids and gases that are opaque in particular bands. There's no reason to question talking about the speed of light in solids.

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I have the October 1980 issue of Scientific American magazine.
The cover story is 'The Causes of Color'.

The article is an overview of the many known causes of color,
including emission, absorption, reflection, dispersion, diffraction,
interference, and others with less-familiar names. Much of the
article is about band gaps in electronic transition levels in atoms.
It is all about how light interacts with matter.

From the article:

It turns out that the ultimate causes of color are remarkably
diverse. An informal classification I shall adopt here has some
14 categories of causes, and some of the categories embrace
several related phenomena. With one exception, however, the
mechanisms have an element in common: the colors come about
through the interaction of light waves with electrons. Such
interactions have been a central preoccupation of physics in
the 20th century, and so it is no surprise that explanations
of color invoke a number of fundamental physical theories.
Indeed, color is a visible (and even conspicuous) manifestation
of some of the subtle effects that determine the structure of
matter.
Another thread in which I quoted more of that article, and lots