# The Naked Scientists Forum

### Author Topic: Does EM of all frequencies travel slower trough matter?  (Read 4340 times)

#### McKay

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##### Does EM of all frequencies travel slower trough matter?
« on: 23/09/2014 09:47:57 »
If so, then classical explenation of photons being absorbed and emited doesnt really make sense, does it?  Say, radiowaves trough non-conductive material - how is it absorbed and emited?

#### JohnDuffield

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##### Re: Does EM of all frequencies travel slower trough matter?
« Reply #1 on: 23/09/2014 10:15:54 »
Maybe that classical explanation is more of a popscience explanation. Another popscience explanation I heard used a pavement analogy. You can walk at 4mph, and on a clear pavement you travel four miles in one hour. But if there's people coming at you, you can't follow a straight path. You aren't bumping into them, but all the weaving around means that whilst you still walk at 4mph, you don't manage to cover four miles in one hour. And then if you're a little guy with short-wavelength steps, you're hindered more than the big guy. Hence in a prism, blue light is refracted more than red light.

#### McKay

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##### Re: Does EM of all frequencies travel slower trough matter?
« Reply #2 on: 23/09/2014 14:57:12 »
Awesome. I like understanding (or trying to understand).
popscience explanation, eh? Well, what is the science-science explanation?
When you bring up the point about "short-wavelength steps", I can understand that, but : 1 meter radio wave traveling trough 1 cm piece of  glass  - would it be slowed down at all? Or a 500nm green-ish wave traveling trough 10nm layer of ..something.
What then is exactly what causes EM radiation to slow down/ walk a longer path. That is, its not like the space-time itself is [THAT significantly] curved around the particles, is it?

#### JohnDuffield

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##### Re: Does EM of all frequencies travel slower trough matter?
« Reply #3 on: 23/09/2014 17:56:54 »
Awesome. I like understanding (or trying to understand). popscience explanation, eh? Well, what is the science-science explanation?
I'm not sure. I was looking for one a few weeks back, and I just couldn't find one. Look on answers.com, and it's the wrong answer. Look on physics stack exchange and they're kind of inconsistent and unsatisfactory.

When you bring up the point about "short-wavelength steps", I can understand that, but : 1 meter radio wave traveling trough 1 cm piece of  glass  - would it be slowed down at all?
I don't think so. It says here that radio waves pass right through glass. But I don't really know myself. All I do know is that you can't always trust things you read on the internet!

What then is exactly what causes EM radiation to slow down/ walk a longer path. That is, its not like the space-time itself is [THAT significantly] curved around the particles, is it?
IMHO no, it's curved space, not curved spacetime. Have a look at  The role of the potentials in electromagnetism by Percy Hammond. See this near the end: "We conclude that the field describes the curvature that characterizes the electromagnetic interaction". Have a google on electromagnetic geometry or electromagnetic curvature. But for some reason you never seem to hear about this sort of thing.

#### chiralSPO

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##### Re: Does EM of all frequencies travel slower trough matter?
« Reply #4 on: 23/09/2014 18:52:36 »
Light interacts with matter in many different ways by virtue of electric and magnetic interactions. Photons can be absorbed and emitted as electrons and nuclei move from one energy level to another (electronic transitions, vibrational transitions, rotational, spin transitions [magnetic nuclei moving between spin states within a magnetic field absorb and emit radio waves]. I don't know which interactions are most important to the dielectric constant and permittivity constant of a material, but I believe it is mostly electron-photon interactions (at least in normal materials--I don't know how that extends to more exotic forms of matter).

#### evan_au

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##### Re: Does EM of all frequencies travel slower trough matter?
« Reply #5 on: 23/09/2014 23:25:27 »
Quote from: McKay
1 meter radio wave traveling trough 1 cm piece of  glass
Electromagnetic waves don't really "notice" dielctric materials considerably thinner than a 1/2 wavelength.
Once the thickness approaches 1/2 wavelength (like a soap bubble), you can get quite intense interference patterns, which shows itself as coloured bands in a soap bubble.

However, in a metallic substance, electrons moving in the metal cancel and reflect the incoming electromagnetic wave - provided the metal is thicker than the skin depth. So a 1cm thick sheet of aluminium would be very effective at reflecting a 1m radio wave.

#### lightarrow

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##### Re: Does EM of all frequencies travel slower trough matter?
« Reply #6 on: 24/09/2014 08:30:56 »
If so, then classical explenation of photons being absorbed and emited doesnt really make sense, does it?
No, it doesn't:
http://www.thenakedscientists.com/forum/index.php?topic=41881.msg371962#msg371962

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lightarrow

#### PmbPhy

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##### Re: Does EM of all frequencies travel slower trough matter?
« Reply #7 on: 24/09/2014 08:49:07 »
Quote from: McKay
Does EM of all frequencies travel slower trough matter?
No. There are a lot of things to consider here. Sometimes EM waves won't travel through material at all. If its an electromagnetic wave which has a fairly low frequency, say lower that microwave, and it impinges on a conducting material then the EM wave cannot travel very deep into it measured by the penetration depth. See
http://en.wikipedia.org/wiki/Penetration_depth

As you can see from that page this idea holds for x-rays entering water too.

When it comes to the actual speed of EM waves traveling through conductors see

It's been a while since I've done those calculations so I'm too rusty to talk about if off the cuff. Sorry.

It's said that when light enters glass then it can travel through it by hoping from one atom to another traveling at the speed of light in a vacuum in between. I don't understand that explanation though since there is no "in-between" atoms in a material like glass.

#### PmbPhy

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##### Re: Does EM of all frequencies travel slower trough matter?
« Reply #8 on: 24/09/2014 14:29:47 »
JohnDuffield is a major crackpot. Beware!!!! [:I]

#### evan_au

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##### Re: Does EM of all frequencies travel slower trough matter?
« Reply #9 on: 25/09/2014 10:24:46 »
Quote
Does EM of all frequencies travel slower through matter?
If "slower" means "no faster"
...and if "slower" means "slower than the speed of light in a vacuum"
....and the speed of EM  means the speed of transmitting information

There are several numbers that are sometimes described as the speed of electromagnetic radiation:
• Phase velocity: For some frequencies, in some materials, this can exceed the speed of light in a vacuum
• Group velocity
• Front velocity
• ...but you can't use these effects to transmit information faster than the speed of light in a vacuum.

There is an explanation here that makes more sense to me than the "absorption and re-radiation" explanation - they describe it as an out-of-phase interfering EM wave induced in the material, generating a complex refractive index:
http://en.wikipedia.org/wiki/Refractive_index#Microscopic_explanation

The absorption & re-radiation explanation may be applicable to experiments where light is "stopped" - and then later released by a laser pulse.

#### lightarrow

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##### Re: Does EM of all frequencies travel slower trough matter?
« Reply #10 on: 25/09/2014 11:07:09 »
The absorption & re-radiation explanation may be applicable to experiments where light is "stopped" - and then later released by a laser pulse.
But in condensed phase it's however a nonsense, for me.
Perhaps my link didn't show immediately the post I was referring to, where I wrote:

Quote

<< Do Photons Move Slower in a Solid Medium?

Contributed by ZapperZ. Edited and corrected by Gokul43201 and inha

This question appears often because it has been shown that in a normal, dispersive solid such as glass, the speed of light is slower than it is in vacuum. This FAQ will strictly deal with that scenario only and will not address light transport in anomolous medium, atomic vapor, metals, etc., and will only consider light within the visible range.

The process of describing light transport via the quantum mechanical description isn't trivial. The use of photons to explain such process involves the understanding of not just the properties of photons, but also the quantum mechanical properties of the material itself (something one learns in Solid State Physics). So this explanation will attempt to only provide a very general and rough idea of the process.

A common explanation that has been provided is that a photon moving through the material still moves at the speed of c, but when it encounters the atom of the material, it is absorbed by the atom via an atomic transition. After a very slight delay, a photon is then re-emitted. This explanation is incorrect and inconsistent with empirical observations. If this is what actually occurs, then the absorption spectrum will be discrete because atoms have only discrete energy states. Yet, in glass for example, we see almost the whole visible spectrum being transmitted with no discrete disruption in the measured speed. In fact, the index of refraction (which reflects the speed of light through that medium) varies continuously, rather than abruptly, with the frequency of light.

Secondly, if that assertion is true, then the index of refraction would ONLY depend on the type of atom in the material, and nothing else, since the atom is responsible for the absorption of the photon. Again, if this is true, then we see a problem when we apply this to carbon, let's say. The index of refraction of graphite and diamond are different from each other. Yet, both are made up of carbon atoms. In fact, if we look at graphite alone, the index of refraction is different along different crystal directions. Obviously, materials with identical atoms can have different index of refraction. So it points to the evidence that it may have nothing to do with an "atomic transition".

When atoms and molecules form a solid, they start to lose most of their individual identity and form a "collective behavior" with other atoms. It is as the result of this collective behavior that one obtains a metal, insulator, semiconductor, etc. Almost all of the properties of solids that we are familiar with are the results of the collective properties of the solid as a whole, not the properties of the individual atoms. The same applies to how a photon moves through a solid.

A solid has a network of ions and electrons fixed in a "lattice". Think of this as a network of balls connected to each other by springs. Because of this, they have what is known as "collective vibrational modes", often called phonons. These are quanta of lattice vibrations, similar to photons being the quanta of EM radiation. It is these vibrational modes that can absorb a photon. So when a photon encounters a solid, and it can interact with an available phonon mode (i.e. something similar to a resonance condition), this photon can be absorbed by the solid and then converted to heat (it is the energy of these vibrations or phonons that we commonly refer to as heat). The solid is then opaque to this particular photon (i.e. at that frequency). Now, unlike the atomic orbitals, the phonon spectrum can be broad and continuous over a large frequency range. That is why all materials have a "bandwidth" of transmission or absorption. The width here depends on how wide the phonon spectrum is.

On the other hand, if a photon has an energy beyond the phonon spectrum, then while it can still cause a disturbance of the lattice ions, the solid cannot sustain this vibration, because the phonon mode isn't available. This is similar to trying to oscillate something at a different frequency than the resonance frequency. So the lattice does not absorb this photon and it is re-emitted but with a very slight delay. This, naively, is the origin of the apparent slowdown of the light speed in the material. The emitted photon may encounter other lattice ions as it makes its way through the material and this accumulate the delay.

Moral of the story: the properties of a solid that we are familiar with have more to do with the "collective" behavior of a large number of atoms interacting with each other. In most cases, these do not reflect the properties of the individual, isolated atoms.>>

So, it's not simply absorption/re-emission. Photon (and of course their properties, included speed) comes from the behaviour of the entire electromagnetic field in the entire system of atoms.

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lightarrow
« Last Edit: 25/09/2014 11:08:48 by lightarrow »

#### lightarrow

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##### Re: Does EM of all frequencies travel slower trough matter?
« Reply #11 on: 25/09/2014 11:27:40 »
About the question: "Does EM of all frequencies travel slower trough matter?"

The answer is yes, but not always. First, as it's been written, it depends on which speed we are talking about, second, sometimes EM radiation travels faster than c in matter!
See for example "anomalous dispersion" and waveguides.

http://en.wikipedia.org/wiki/Group_velocity
<<Anomalous dispersion happens in areas of rapid spectral variation with respect to the refractive index. Therefore, negative values of the group velocity will occur in these areas. Anomalous dispersion plays a fundamental role in achieving backward propagating and superluminal light. Anomalous dispersion can also be used to produce group and phase velocities that are in different directions.[9] Materials that exhibit large anomalous dispersion allow the group velocity of the light to exceed c and/or become negative.[11][12]>>

In waveguides, phase velocity can exceed c.

In both cases, the "real" velocity, that is the speed at which information and energy travels, is neither phase nor group velocity and we have to introduce "signal velocity".
http://www.mathpages.com/home/kmath210/kmath210.htm

This "real speed" can never exceed c.

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lightarrow
« Last Edit: 25/09/2014 11:29:14 by lightarrow »

#### yor_on

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##### Re: Does EM of all frequencies travel slower trough matter?
« Reply #12 on: 26/09/2014 08:15:34 »

#### The Naked Scientists Forum

##### Re: Does EM of all frequencies travel slower trough matter?
« Reply #12 on: 26/09/2014 08:15:34 »