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SRH
2012-Jul-08, 03:25 PM
Hi guys,
A bunch of questions here and I would really appreciate your help...


...if you could hypothetically shine a light beam at a certain frequency through space and it passes through either a giant pool of water floating thorough space, or through a giant cloud of hydrogen gas...

1. would the speed of light slow down as it passes through the medium? (since it is no longer in a vacuum when it is in the medium)?
2. when the light exits the medium, and enters the vacuum again, i assume the light speed returns to C again.

3. would the light redshift inside the medium? provided you had a detector inside the medium (pool of water or hydrogen gas cloud)
4. if the light does redshift inside the medium (i'm not saying that it does), would it then blue shift when it exits the medium and re-enters the vacuum in space?
or would it stay at the redshift that it obtained in the medium upon exiting the medium and entering the vacuum again? (to be clear, I am not talking about scattering here)

5. Does light that is scattered show any redshift? or is it still the same frequency post-scattering, just propagating in different directions (i.e. blurry)?

6. Someone posted this in another thread in answer to my question..."What factors influence the red shift in electromagnetic radiation?"
Answer: "Gravity, proper motion and cosmological expansion"
Could someone please explain what "proper motion" means? (I should probably ask this question in the other thread, so I apologize, and it is okay if you don't answer me here).


Thanks so much for your help!!

caveman1917
2012-Jul-08, 03:37 PM
1. would the speed of light slow down as it passes through the medium? (since it is no longer in a vacuum when it is in the medium)?

Yes, see "index of refraction".


2. when the light exits the medium, and enters the vacuum again, i assume the light speed returns to C again.

Yes.


3. would the light redshift inside the medium? provided you had a detector inside the medium (pool of water or hydrogen gas cloud)

No, its frequency would change but not its wavelength. Because the wave is travelling slower it takes a longer time for the second crest of the wave to arrive after the first one (ie lower frequency), but the distance between the crests remains unchanged (so no change in wavelength).


5. Does light that is scattered show any redshift? or is it still the same frequency post-scattering, just propagating in different directions (i.e. blurry)?

That depends on the type of scattering. Elastic scattering (what you would usually consider scattering) doesn't change the wavelength. Inelastic scattering, in which the photon interacts with the medium it is scattered in, does (because the photon can lose or gain energy with that interaction).


6. Someone posted this in another thread in answer to my question..."What factors influence the red shift in electromagnetic radiation?"
Answer: "Gravity, proper motion and cosmological expansion"
Could someone please explain what "proper motion" means? (I should probably ask this question in the other thread, so I apologize, and it is okay if you don't answer me here).

It means the relative motion between source and observer, by the doppler effect. Not the relative motion due to cosmological expansion (say a distant galaxy moving away), but that due to simple local motion, say a comet coming towards us.

antoniseb
2012-Jul-08, 03:42 PM
...Could someone please explain what "proper motion" means? ...
Proper motion in this context is simply motion relative to the frame of reference (the usual local idea behind Doppler shift).
When we talk about nearby stars, the term proper motion usually refers to only the motion in the plane perpendicular to our line of sight, and so it is measured in milliarcseconds/century or similar units, while the motion along our line of sight in that context is called radial velocity.
Outside that particular context, we mean simply motion relative to the observer... though when looking cosmological distances, we distinguish between "motion from the expansion of space" and "proper motion", where two particular galaxies in the same cluster (for example) may have very different motions relative to us, one orbiting the center toward us, and the other going away...

chornedsnorkack
2012-Jul-08, 03:44 PM
No, its frequency would change but not its wavelength. Because the wave is travelling slower it takes a longer time for the second crest of the wave to arrive after the first one (ie lower frequency), but the distance between the crests remains unchanged (so no change in wavelength).

Nonsense!

Wavelength changes - frequency is unchanged.

Just consider the waves on either side of a surface. Waves behind the surface move because of the waves arriving in front of tjhe surface - therefore they have to be at the same frequency.



That depends on the type of scattering. Elastic scattering (what you would usually consider scattering) doesn't change the wavelength. Inelastic scattering, in which the photon interacts with the medium it is scattered in, does (because the photon can lose or gain energy with that interaction).


Where does the momentum difference of the scattered photons go?

caveman1917
2012-Jul-08, 03:56 PM
Nonsense!

Wavelength changes - frequency is unchanged.

Just consider the waves on either side of a surface. Waves behind the surface move because of the waves arriving in front of tjhe surface - therefore they have to be at the same frequency.

Yes you're right, i had it backwards there.


Where does the momentum difference of the scattered photons go?

Possibly emitted in a second photon as the atom goes back to its ground state.

korjik
2012-Jul-08, 04:14 PM
Or into momentum of the electron it interacted with, if the electron is free.

SRH
2012-Jul-08, 09:36 PM
Nonsense!

Wavelength changes - frequency is unchanged.

Just consider the waves on either side of a surface. Waves behind the surface move because of the waves arriving in front of tjhe surface - therefore they have to be at the same frequency.


Where does the momentum difference of the scattered photons go?


Just to clarify here...

When the wavelength changes as it enters the medium, it red-shifts, correct?
And does the light beam blue-shift upon exiting the medium and re-entering the vacuum?
(Red and blue shifts are always determined by the wavelength and not the frequency, correct?)

On a related topic...
When we say that C= wavelength x frequency...is this a proportional relationship? or does the equation predict specific mathematical outcomes?
What I am trying to ask is can we say mathematically that 10= 2 x 5 (#lightspeed = #wavelength x #frequency)?, or can we only say directionally that "if C is constant and the frequency increases, then the wavelength must shorten"?

Also, does the equation (C=wavelength x frequency) hold always, or only in a vacuum?

Thanks for all the help!

Hornblower
2012-Jul-08, 11:15 PM
It is my understanding that when light enters a medium such as water or glass, the wavelength is diminished in proportion to the reduced speed of propagation, so the frequency remains the same.

Redshift or blueshift refers to changes in the energy per photon for reasons independent of the presence of any medium, specifically gravity, relative motion of emitter and detector, cosmic-scale expansion of space, or some combination thereof. This energy level corresponds to changes in the wavelength and frequency, which in any given environment are inversely proportional to one another.

WayneFrancis
2012-Jul-09, 12:39 AM
Just to clarify here...

When the wavelength changes as it enters the medium, it red-shifts, correct?

No


And does the light beam blue-shift upon exiting the medium and re-entering the vacuum?
(Red and blue shifts are always determined by the wavelength and not the frequency, correct?)

No, All the photons do is return to c.




On a related topic...
When we say that C= wavelength x frequency...is this a proportional relationship? or does the equation predict specific mathematical outcomes?
What I am trying to ask is can we say mathematically that 10= 2 x 5 (#lightspeed = #wavelength x #frequency)?, or can we only say directionally that "if C is constant and the frequency increases, then the wavelength must shorten"?

The frequency doesn't change. Blue light stays blue light. Now this is if you are still talking about a medium.
Now if you are talking cosmological expansion then you can think of it as both the frequency and wavelength changing which causes the lowering of energy in red shift and the increase of energy in blue shift. waiting for chornedsnorkack to correct me here if I've got that wrong :)



Also, does the equation (C=wavelength x frequency) hold always, or only in a vacuum?

Thanks for all the help!

well, lower case 'c' or the speed of light in vacuum is the speed of light in a vacuum. Don't confuse the issue with the speed of light in a medium. Tho I hear people use c in other mediums, and I've undoubtedly used the expression myself, I don't think it is very good because it alters the definition of c. That said the speed of light through a medium, not counting absorption and emission, is c=λv where λ = wavelength and ν = frequency . Another issue I have here is I'm not sure the standard model predicts that a graviton would be slowed by a medium or not, gluon not actually travelling far enough to take mediums into account.

a1call
2012-Jul-09, 01:03 AM
Just to clarify here...

When the wavelength changes as it enters the medium, it red-shifts, correct?
And does the light beam blue-shift upon exiting the medium and re-entering the vacuum?
(Red and blue shifts are always determined by the wavelength and not the frequency, correct?)



The speed of light in any given medium is independent of the speed of light in any previous medium the light has passed through. For example light entering any number of transparent sheets (ie parallel faces) will exit at the same angle that it enters them. This is despite the fact that light refracts (changes direction) while inside the sheets.
White light entering a thick layer of glass will exit at the same angle as the incident angle but will shift in position. Different wavelengths will shift differently and one side will be blue edged and the other red edged.

SRH
2012-Aug-04, 06:08 PM
Does longer wavelength light lose more speed than higher wavelength light, in the same medium? Does the speed loss depend on the wavelength of the light as it enters the medium?
This may or may not be the same question/answer, but...Does the speed loss depend on the frequency of the light as it enters the medium?
Thanks.

ctcoker
2012-Aug-04, 06:56 PM
Does longer wavelength light lose more speed than higher wavelength light, in the same medium? Does the speed loss depend on the wavelength of the light as it enters the medium? That depends on the specific properties of the medium, and also what portion of the spectrum you're looking at. You should look up the concept of dispersion. Long story short, in virtually all media, the index of refraction depends on the frequency of the light (this is why a prism can split white light into colors).

SRH
2012-Aug-04, 07:23 PM
Thanks.

WayneFrancis
2012-Aug-04, 07:58 PM
I believe the speed is the same but because the angle of refraction is dependant on the frequency of light the path within the medium can be longer or shorter

ctcoker
2012-Aug-04, 08:17 PM
No, the speed is different as well (v = c/n).

Jens
2012-Aug-06, 12:11 AM
Just as a clarification, my understanding is that c does not really change, that photons are only capable of traveling at c and no other velocity. So the difference of speed in a medium is not because there is any change in the speed of light, but because the photons have a chance of being absorbed and then emitted again by atoms along the way. Is this correct?

ctcoker
2012-Aug-06, 12:44 AM
Yes, that's right. Photons exist only at c, but the macroscopic wave of photons slows down due to exactly what you describe. I also wouldn't really describe it as a "chance" that the photons can be absorbed by atoms, except for very tenuous media. In everyday cases like glass, water, and air, the photons will strike many atoms before they reach your eyes.

korjik
2012-Aug-06, 06:13 AM
nitpicking definitions, 'c' is the universal constant known as the speed of light in a vacuum. The speed that a beam of light actually goes is c/n where n is the index of refraction of the material being traversed. It can be alot less than 300000 kps. Interestingly tho, a photon, as opposed to the bulk speed of a beam, will always travel at c.