Does light slow down in space?

[jimmydasaint]

My Physics knowledge is not good but can someone answer these questions.

 

Space is not a perfect vacuum - does light not slow down in the space dust and gas particles in space?

 

In a piece of glass or water, does the light slow down when it enters at an angle of 90 degrees to the surface of the medium as it does when it enters at an angle?

 

Why is light constant when something approaches or hits it at speed? For example if you travel at 86,000 miles per second why is the effective speed of light not 186,000-86,000 miles per second?

 

Thanks

 

[swansont]

Yes, light will slow down, but space is a pretty good vacuum, so its index of refraction is going to be really, really close to 1, so there's only a tiny effect on the speed. Light also scatters, which is why we can't see some things — too much dust is in the way.

 

The propagation speed of light is how nature behaves. Why it behaves that way is more a question of philosophy or metaphysics.

[Klaynos]

Each photon does not slow down, but gets absorbed and re-emitted. This does happen in space, but as the material is so spaced out the probability of ab absorption event is tiny....

[jimmydasaint]

As you have sussed out, my knowledge of Physics does not extend beyond GCSE or K11. However, does light also slow down at 90 degrees to a surface when it passes straight through? It should do, shouldn't it? Also light does not penetrate down to the bottom of the sea so it presumably is absorbed beforehand.

[Klaynos]

Yes, and yes The energy from the light goes into heating up the water

[jimmydasaint]

Which leaves me with one question which is puzzling. Why does the combined speed of light and another object which hits it not equal the sum of their speeds. I mean, if two cars travelling at 70mph collide, the combined speed is 140mph. Why should it be different for electromagnetic waves?

[Klaynos]

That's a bit harder to explain.

 

It comes down to two thngs being true:

 

The speed of light is constant, if you travel at 100mph compared to a stationary observer and fire a photon, it'll travel at c compared to a stationary observer, if you travel at 1000mph compared to a stationary observer and fire a photon it'll still travel at c compared to a stationary observer. This has been experimentally tested. And comes from electromagnetism.

 

And that the laws of physics are constant in all reference frames.

 

Working from them you can derive the special theory of relativity that shows you that you can't just add up velocities that are close to the speed of light like you can classically. Infact the two cars wont have a velocity measured by the other one of 140mph, but the difference will be so amazingly tiny that you can't possibly measure it.

[thedarkshade]

Except the difference in the addition of velocities, measuring relative kinetic energy differs from the classical formula for kinetic energy.

[Kyrisch]

The index of refraction of air is so close to 1 that it is usually estimated to be that in calculations which just goes to show how much closer to 1 the index of refraction would be for a near-vacuum.

[vab]

Einstein proved that the speed of light is the only thing in the universe that is constant, everything else is relative to something else and is in turn effected by everything else. this is why in his equation c is the speed of light which also stands for constant, if light slows down it is no longer light.

[MrSandman]

Hate to break vab's bubble, but light is affected by gravity and can be slown down by atmospheres, and other things. You guys can also look up about the speed of light slowing down, whether or not it's real...IDK!

[Gilded]

People are confusing the constant c with the "other" speed of light again. Sometimes I think it would be less of a hassle if c was referred to as "photonspeed" or something. As mentioned photons travel at c, where as light as a phenomenon, a "wavefront" if you will, can travel slower as the individual photons get absorbed and re-emitted. To actually add something rather than just repeat what has already been said, photons are indeed affected by gravity but it doesn't accelerate or slow them down.

[D H]

Photons travel at c in vacuum. They travel at a lesser speed in some physical medium. Assuming light always travels at c is an error that has been repeatedly made in this thread, for example

Each photon does not slow down, but gets absorbed and re-emitted. This does happen in space, but as the material is so spaced out the probability of ab absorption event is tiny....

 

We see stars as stars -- pinpoints of light with the spectrum expected from a star's photosphere) even when the light passes through our atmosphere or through a piece of glass. Absorption and reemission do not explain this. It is the speed of light itself that varies when the light travels through a physical medium. While the speed of light in air is only a tiny bit less than the speed of light in vacuum, the speed of light in glass is about 2/3 c.

 

A physical medium does not have the same permittivity and permeability as does a pure vacuum. The speed of light is a direct consequence of permittivity and permeability, and the change in the speed of light in a physical medium results in refraction. Without refraction we would not have microscopes, telescopes, or eyeglasses.

[Klaynos]

Photons travel at c.

 

The apparent slow down is due to absorption and emission. That's the physica reality of it.

 

If the above was false then EM would not work...

 

With stars, even the dust in space makes a difference which has to be taken into account when calculating the luminosity,

 

Oh and of course you'd need a mechanism to slow the photon down, which you can't do because photons travel at c they are timeless...

[swansont]

Photons travel at c in vacuum. They travel at a lesser speed in some physical medium. Assuming light always travels at c is an error that has been repeatedly made in this thread, for example

 

 

We see stars as stars -- pinpoints of light with the spectrum expected from a star's photosphere) even when the light passes through our atmosphere or through a piece of glass. Absorption and reemission do not explain this. It is the speed of light itself that varies when the light travels through a physical medium. While the speed of light in air is only a tiny bit less than the speed of light in vacuum, the speed of light in glass is about 2/3 c.

 

A physical medium does not have the same permittivity and permeability as does a pure vacuum. The speed of light is a direct consequence of permittivity and permeability, and the change in the speed of light in a physical medium results in refraction. Without refraction we would not have microscopes, telescopes, or eyeglasses.

 

 

One needs to be clear about whether the classical or quantum explanation is being used. Classically, it is the variation of the permeability and permittivity that slow the wave speed down , and we say that light slows down in a medium. Quantum-mechanically it is the absorption and re-emission of photons by virtual states in the medium. The photons travel at c, but it takes time to absorb and re-emit them. The "speed of light in a medium" and the "speed of a photon" are not the same thing.

 

Difficulties arise when you try and mix the two explanations together. They aren't miscible.

[D H]

Photons travel at c.

 

The apparent slow down is due to absorption and emission. That's the physica reality of it.

 

If the above was false then EM would not work...

 

EM "works" precisely because photons travel at less than c in a physical medium.

 

http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/speed_of_light.html

Does the speed of light change in air or water?

Yes. Light is slowed down in transparent media such as air' date=' water and glass. The ratio by which it is slowed is called the refractive index of the medium and is always greater than one.* This was discovered by Jean Foucault in 1850.

 

When people talk about "the speed of light" in a general context, they usually mean the speed of light in a vacuum. This quantity is also referred to as c.

[/quote']

http://www.physicsforums.com/showthread.php?p=899393#post899393

A common explanation that has been provided is that a photon moving through the material still moves at the speed of c' date=' 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. [b']This explanation is incorrect and inconsistent with empirical observations.[/b] 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".

[Klaynos]

Photons travel at c.

 

Physics forums FAQ, I'm afraid is not a technical resource.

[D H]

Please. Now you are sounding like a crackpot.

 

If you want a technical resource that describes why the speed of light is reduced in a physical medium from a quantum mechanical perspective I suggest you take an upper level undergraduate solid state physics course. That the reduction in the speed of light results from absorption and reemission is a common misperception based on the fact that space is largely empty even in a physical material. A photon should go at the speed of light between atoms, shouldn't it? This is not what happens. Even though the space is largely empty, the atoms/molecules have an electromagnetic reach that spans the intermolecular space. This field slows photons down without any absorption.

 

Absorption is a quantum phenomenon. Atoms/molecules can only absorb photons of a very specific wavelength. If absorption/reemission were the cause of the reduction of the speed of light in a material, the speed of light through a material would depend very strongly on frequency. Most frequencies of light would pass through the material unaffected by the material. That is contrary to all experimental evidence. The index of refraction does depend on frequency, but it is a very smooth function of frequency.

[Klaynos]

Interestingly I've done a postgraduate course in solid state physics and another in quantum mechanics...

 

And you know what they tell me, you're wrong and that photons ALWAYS travel at c.

 

Considering the absorption of individual atoms in a material is such a gross simplification you can't apply it.

[swansont]

A common explanation that has been provided is that a photon moving through the material still moves at the speed of c' date=' 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. [b']This explanation is incorrect and inconsistent with empirical observations.[/b] 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".

 

Note the emphasis on "atomic transition," which is referring to two distinct, real states of an atom or system. That's not what is under discussion. Virtual states are not real states. A transition in the medium involving a virtual state requires that the outgoing photon have the same energy and momentum as the incoming photon.

[Mr Skeptic]

If you consider a classical electromagnetic wave traveling near/through classical electrons, then wouldn't the wave equation be slightly different? The electrons would get moved a tiny bit by the EM fields of the wave, so that the wave equation in space would no longer apply. A different equation could give a different propagation speed in accordance to Maxwell's Equations. Not sure how this would work with electron orbitals, though.

 

Am I close?

[swansont]

Classically, that's the description of happens, and what D H was referring to earlier.