# Thread: Speed of Light Q's

1. ## Speed of Light Q's

Okay, some questions about the speed of light:

If I'm not mistaken, the speed of light slows down when passing through water. That's the explanation I've been given as to why straws look bent in a water glass.

First question: Is that correct?

Second question: If the first question is correct, what is the speed of light through water?

Third question: Does light decrease speed in our atmosphere compared to space's vacuum?

Fourth question: If a photon is skimming through space at the speed of light, then slows down as it passes through, say, Earth's atmosphere, does it speed up after exiting the atmosphere back into space? If so, what speeds it back up?

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1) Correct

2) I don't know.

3) Yes.

4) AFAIK, photons always travel at c, aka "the speed of light in a vacuum". The slowing of light through a physical medium is a result of the atoms of that medium absorbing and re-transmitting photons a finite time afterwards.

So it's not that the photons are being "sped up" as they leave the medium, it's just that they're no longer "pausing" each time they hit an atom.

(Strictly speaking, they don't pause - They are absorbed on interacting with the atom's electron shell, exciting electrons in the process. When these electrons return to an unexited state, they realease a new photon that has the same frequency as the original.)

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2) The speed of light in a transparent medium is just c divided by the refractive index of the medium.
Water has a refractive index of 1.33, making the speed of light in water about three-quarters of its speed in a vacuum.

3) The slight reduction of the velocity of light in air is what accounts for mirages: the denser the air, the slower the light, and so we get refraction whenever the density of the air varies along the path of a light ray.

4) The business of absorption and retransmission involves something called the "dressed state" of the atoms in the medium. If an atom has an absorption line at a particular energy, it can temporarily absorb photons of lower energy by "borrowing" the missing energy under the terms of Heisenberg's uncertainty principle. But once the time limit on the borrowed energy elapses, the photon must pass on its way again.
Most transparent mediums have ultraviolet absorption lines just above the energy of the visible spectrum, so they retard the passage of visible photons by this mechanism. Since blue photons have energies closer to the UV than do red photons, the blue photons can participate in the dressed state for longer than red photons - the energy required from Heisenberg is lower for the blue photons, and so it can be retained for longer.
So refractive mediums generally retard blue photons more than red, and therefore have a higher index of refraction for blue light than red light.

Grant Hutchison

4. Originally Posted by jamesabrown
Fourth question: If a photon is skimming through space at the speed of light, then slows down as it passes through, say, Earth's atmosphere, does it speed up after exiting the atmosphere back into space? If so, what speeds it back up?
It just does; photons in vacuum have a fixed speed, but it has lost energy in the process and is at a lower frequency. So the laws of thermodynamics are unbroken.

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Originally Posted by Stuart van Onselen
(Strictly speaking, they don't pause - They are absorbed on interacting with the atom's electron shell, exciting electrons in the process. When these electrons return to an unexited state, they realease a new photon that has the same frequency as the original.)
Correct. And to boot, they posess the original's vector, as well.

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Originally Posted by mugaliens
Correct. And to boot, they posess the original's vector, as well.
I was unsure about that one. If that's true, what causes scattering and reflection?

(Or maybe I should just look it up myself, there should be some good quantum physics guides on the net.)

7. Originally Posted by Stuart van Onselen
1) Correct

2) I don't know.

3) Yes.

4) AFAIK, photons always travel at c, aka "the speed of light in a vacuum". The slowing of light through a physical medium is a result of the atoms of that medium absorbing and re-transmitting photons a finite time afterwards.

So it's not that the photons are being "sped up" as they leave the medium, it's just that they're no longer "pausing" each time they hit an atom.

(Strictly speaking, they don't pause - They are absorbed on interacting with the atom's electron shell, exciting electrons in the process. When these electrons return to an unexited state, they realease a new photon that has the same frequency as the original.)
Sorry about dropping out of sight; death in the family forced me away from a networked computer for a week.

Thanks for the explanation; I get it now. I was under the impression that the photon 'bored' through the water/air/whatever medium and that on exiting it sped back up to c. Now I understand that it has to collide with all the atoms first, just like a person crossing a crowded room can't walk as fast as he can down an empty hallway.

So I presume that when a photon strikes, say, a brick wall, it doesn't excite an electron enough to generate a new one. But wait . . . doesn't the brick grow warmer and cool off in the infared? Isn't the infared light composed of photons, just traveling at c with a longer wavelength?

8. Originally Posted by grant hutchison
2) The speed of light in a transparent medium is just c divided by the refractive index of the medium.
Water has a refractive index of 1.33, making the speed of light in water about three-quarters of its speed in a vacuum.
Cool. Does everything have a refractive index? Is a vacumn's refractive index 1? or 0? Is there an upper limit?

9. Originally Posted by mugaliens
Correct. And to boot, they posess the original's vector, as well.
I don't get that. If a photon is heading straight for Mars, but Earth's atmosphere gets in the way, I understand that the photon will random walk throughout the air, getting absorbed and re-emitting all the way, but how does the last photon emitted know to eventually exit the Earth's atmosphere and head for Mars again?

10. It doesn't-- the only process that preserves the original vector is stimulated emission, like in a laser.

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Originally Posted by jamesabrown
Cool. Does everything have a refractive index? Is a vacumn's refractive index 1? or 0? Is there an upper limit?
Everything transparent has a refractive index. I'm not sure if it would have any meaning for something that was completely opaque.
Vacuum has a refractive index of 1. Air's refractive index varies with its density, and is just a tad more than one. Diamond is considered to have a high refractive index at 2.4. I don't know if there is an upper limit.

Grant Hutchison

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