Science a GoGo's Home Page Forums General Discussion General Science Discussion Forum We accept Light Slows Down So what makes it Speed Up?

Its accepted that a light beam slows down when passing thru a Glass block.
So what makes it speed up upon exiting the glass?
Even more strange when you realise that the Light beam loses energy in heating up the glass.

And it is accepted that sound waves go faster through glass than therough air. So what makes a sound wave speed up in glass? So this is not an issue pertaining only to how liht behaves, it might be more general.

As for the even stranger fact that for light passing through a glass it looses enrergy, this is actually the simplest question.A beam of light loosing energy is not the same as a photon losing energy.When a beam of light looses energy, this is because its intensity decreases, i.e. some of the photons are absorbed by the glass. Nothing happens with the frequency of the photons, only with their number.

Mike ... take a good look at the December issue of Scientific American ... there is a great article on a subject on light.

I have always thought the Michelson-Morley experiment bogus and this article is in line with my thinking.

There are plenty of examples of a fluid not creating friction. That wasn't true when MM did their work but it certainly is today.

Hi Dan, thanks, as yet I hav'nt seen the Dec: issue. Like yourself I have always had reservations re the MM experiment. In my mind I have always assumed that the experiments were not sensitive enough. Or that the 90 degree 'turn' involved, affected the length of the direction, and the measuring equipment, by exactly the same amount.
But from the scientific point of view, since no effect was found, one has to accept that there is no Aether. 'Illogical' as Dr Spock might say.

Prehaps it's tied up with the ability of light to alter its speed thru different mediums? Or even arriving from different directions. Measurements which even themselves out, once you change their directions on the lab turntable?

I appreciate Pasti's statement above, re losing a few photons (to heat)
I still cannot really understand as to why the photons speed up to their original state, regardless of frequency.
Wouldnt photon frequency get lower travelling thru a thicker medium. Yet red light travels at the same speed as blue? If you see what I mean.

Lightspeed is invariant for all inertial observers. Refractive index arises from repeated absorption and re-emission within a medium re the Kramers-Kronig relationship. Refractive index has a real part (n) arising from phase velocity and an imaginary part (k) arising from absorption. Light does not "slow down" and "speed up."

Ordinary refractive index is roughly proportional to the square root of medium dielectric constant (polarizability of the medium) at frequency. To get fast signal transfer on a circuit board or in a chip you want the wire to be surrounded by the smallest possible dielectric constant or polarizability, hence the desirablity of fluorinated organics like teflon insulation or holey silicas.

Bushwa about "superfast optical computers" is just that. Electrical pulses propagating in a signal line and light pulses propagating through a fiberoptic travel at almost exactly the same speed.

The vacuum has a refractive index of eactly 1, no dispersion (refractive index varying with frequency), and manifests no magnetooptical or chiroptical effects to several orders.

In principle one could trivally exceed lightspeed for light propagation by diddling with the permeability and permittivity of the vacuum, hence the never-observed Scharnhorst effect,

http://arXiv.org/abs/gr-qc/0107091
http://arXiv.org/abs/quant-ph/0010055
Phys. Lett. B236 354 (1990)
Phys. Lett. B250 133 (1990)
J Phys A26 2037 (1993)

Mike asks:
?Why does a beam of light slow down when passing through glass?

I will expose myself to the wrath of the powerful by changing the question to some thing more basic and even more important. How does light, a photon of energy travel a million light years so we can see it here at Earth? Why does the photon travel at all? Are we looking at a shot of energy that almost never wears out (except for the inverse square rule) and goes, and goes, etc?

It will not be provable here but I have long ago contended that light travels by expansion, similar to a gas in expansion, but not the same because an expanding gas loses density rapidly and dissipates while the photon does not.. With this idea the light passing through glass or other substances slows down because it can not express its normal expansion which is the source of its speed. This is much too simple and idea for dedicated physics promoters.

Let?s go back even farther to the origin of light. Mainstream science insists that all light, from all sources, blasts off at the same speed it always maintains, (with the possible exception of mediums that may affect that speed temporarily). I am well aware that those that know advise there is no place for every day logic when it comes to scientific fact- but I rebel at the suggestion light would blast off instantaneously at 186,280 miles a second and always maintain that speed everywhere and continuously Gravity is considered a possible medium effecting light but not in an every day situation such as the Earth?s environment, and why not? Can we comprehend that light can be slowed in a gravitationally specific location just like in glass or water? Not yet, it seems. Before I am asked, I offer that my insights into this area are not supported by any Academic Degrees, Just a lot of attention to the issue.

jjw

Mike:"I still cannot really understand as to why the photons speed up to their original state, regardless of frequency.Wouldnt photon frequency get lower travelling thru a thicker medium. Yet red light travels at the same speed as blue? If you see what I mean."

I know what you mean, don't worry. And the explanation is rather straightforward, if you think about it. I will make the analogy with a skier. Take a skier in a downhill race containing three legs of equal lengths: a first leg without gates, a second leg with gates, and a third leg also without gates. Now assume that somehow, through all these legs of the race the skier goes with the same speed.
While he races through the first leg, he goes straight downhill,but when he enters the leg with the gates he will have to make turns to avoid the gates. Even though he goes with the same speed, it will take him longer to clear this leg with the gates, because in order to avoid the gates, he has to make a lot of turns, hence he is actually travelling a larger distance. When he enters the final leg, he will once again go straight downhill.

Now, to someone who clocks the skier only at the ends of each leg, and cannot actually see the race course, it will appear that the skier goes fater through the first leg, slows down through the second, then speeds up through the third. Sounds familiar?

Same thing happens, (very) roughly speaking with photons in matter. When photons enter the glass, they have to "avoid" the atoms in glass, and hence they will have to make "turns" in order to do that. The net effect appears to be that photons slow down (light slows down) when in fact the photons are just travelling a larger distance than what we think it is (matter forces light to travel on a longer path), at the same speed as in vacuum. Hope this helps.

Hi Pasti:

These gates to which you refer, are they created by inference to explane a result that otherwise lacks a determined explanation or are they logically assumed to describe a result that we have no positive account of? Your conclusion of the longer time requirement is undoubted based upon your hypothetical.
jjw

A nice explanation, and one needs to fully realise that this turning doesn't slow the photons as it might the skier.

Dogrock:

Pasti is not suggesting any slowing by turning or the like. She suggests more distance covered taking more time to travel the course with the gates than without. There is nothing wrong with the anology for skiers.

We must now decide what kind of gates the glass block has for photons?
jjw

Absorption and re-emission. Kramers-Kronig relationship.

Al:"Absorption and re-emission. Kramers-Kronig relationship."

Al, don't confuse the people. First of all, the Kramers-Kronig relations are useless unless you know either the real or the imaginary part of the electric permitivity. And for one thing, the electric permitivity is a classical concept, while the photon is a quantum concept. For a second thing, even if you want to use a semiclassical cross-section for absorption, you also need the magnetic permitivity.

As for emission, good luck with calculating decay rates based on classical electrodynamics. Emission is a pure quantum concept, and this is the reason why you cannot go beyond semiclassical rate equations when it comes to emission.

BTW, since you like simulations. Take a 3-d crystalline material, known, say NaCl, or something for which you can find the full band structure in the ASTM tables. Consider photons near some band transition, and assume they travel through your crystal using the absorption/re-emission mechanism you propose. Using just rate equations, with the relavant parameters from the tables, calculate the time necessary for a photon to travel through a given length of the material. Then compare this with the speed of light in the material as calculated from the refractive index of the material at that frequency (the moral of the story is that you will get unreasonably small velocity for the photon if you have absorption/re-emission mechanisms, because the decay times are rather large).

And this is not all. What about the materials transparent to certain frequencies? Where do you have there absorption and re-emission for radiation in the transparency domain?

The explanation is much simpler: the "gates" are usually the atoms of the material. Since photons are quanta of electromagnetic field, once again, roughly speaking, the electromagnetic field of the photon (the skier) interacts with the electromagnetic field of the atom (the gate), and the photon is (very mildly) scattered, "repelled" by the atom towards another atom, and so on and so forth.
A full explanation would require quantum electrodynamics, and I doubt the full theory would do any good to anyone in this context.