Consequences of light moving in a medium

In summary: This is due to the fact that it is interacting with the atoms in the medium, which take time to absorb and re-emit the light. This effectively slows it down, resulting in a lower speed. However, it is still moving at the constant speed of light in the vacuum, but it is just taking a longer path due to the interactions with the medium.
  • #1
Kiley
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I'm reading Special Relativity by TM Helliwell and in it he describes the second postulate and the fact that moving with respect to air changes the speed of sound, and that because light doesn't need a medium it's speed is constant. I remember my physics teacher saying that light itself(EM fields) is the medium in which it travels and I would like to know if that is true and what the consequences of that being true are in SR. I would also like to know if c truly changes in mediums or if it is just traveling a longer distance because I've heard both. Thank you for your time
 
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  • #2
Kiley said:
I remember my physics teacher saying that light itself(EM fields) is the medium in which it travels and I would like to know if that is true and what the consequences of that being true are in SR.
It is not true as far as I know. At least, I am unaware of any reference which describes it that way or investigates the consequences of such a claim.
 
  • #3
Ok, thank you for your reply Dale.
 
  • #4
Kiley said:
I'm reading Special Relativity by TM Helliwell and in it he describes the second postulate and the fact that moving with respect to air changes the speed of sound, and that because light doesn't need a medium it's speed is constant. I remember my physics teacher saying that light itself(EM fields) is the medium in which it travels and I would like to know if that is true and what the consequences of that being true are in SR. I would also like to know if c truly changes in mediums or if it is just traveling a longer distance because I've heard both. Thank you for your time

In general, I'd say the best thing is to forget everything you thought you previously knew about SR. Helliwell's book is excellent, in my opinion, and self-contained. If it contradicts in any way something you thought you knew, go with Helliwell and assume that what you previously knew must have been mistaken in some way.

That said, you may be missing his point about the difference between sound and light. Because sound needs a physical medium (such as air), it has a defined speed with respect to that medium, but the observer can be moving relative to that medium. You can analyse the motion of the sound waves relative to you as a) the constant speed of sound in air; and, b) your motion relative to the air.

Light, however, can move though the vacuum of space. And, because this is not a physical medium and you cannot detect your motion relative to space (see the discussion on the search for the imaginary ether), then the speed of light in a vacuum must be constant for all observers. This is fundamental to SR.

Finally, light can move through a transparent medium (such as water or glass). In this case, the motion of light is disrupted by the medium as light interacts with the medium itself. In this respect, the speed of light in such a medium is not ##c##. This has no bearing on SR, because it's the property of light moving through a vacuum that is the key issue. The fact that light can also move through a physical medium as well as a vacuum is not relevant to the core argument.
 
  • #5
PeroK said:
Finally, light can move through a transparent medium (such as water or glass). In this case, the motion of light is disrupted by the medium as light interacts with the medium itself. In this respect, the speed of light in such a medium is not ##c##. This has no bearing on SR, because it's the property of light moving through a vacuum that is the key issue. The fact that light can also move through a physical medium as well as a vacuum is not relevant to the core argument.

I'd also add that there's a semantic issue that arises when we talk about "light" traveling through a medium.

I'm no expert, but my understanding is that the wave propagating through the medium is actually a superposition of the incoming radiation and the electromagnetic waves emitted by the excited atoms in the material. According to the extinction theorem, this superposition results in some canceling that leaves a polarization wave whose phase velocity is lower than ##c##. The semantic issue is whether to call this polarization wave "light."

(And then there are some people who reserve the word "light" for EM waves in the visible spectrum...)
 
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  • #6
Kiley said:
I would also like to know if c truly changes in mediums or if it is just traveling a longer distance because I've heard both.

The speed of light truly changes in a medium. It needs more time for the same distance.
 
  • #7
Thank you all for your replies, they have all been very helpful.
 

1. What is the speed of light in a medium?

The speed of light in a medium depends on the properties of the medium, such as its density and refractive index. In general, the speed of light in a medium is slower than the speed of light in a vacuum, which is approximately 299,792,458 meters per second.

2. How does light travel through different mediums?

Light travels through different mediums by interacting with the particles in the medium. When light enters a medium, it is absorbed and re-emitted by the particles, causing it to travel at a slower speed. This process is known as refraction.

3. What are the consequences of light moving in a medium?

The consequences of light moving in a medium include refraction, dispersion, and absorption. Refraction causes the light to change direction when it enters a different medium, dispersion causes the separation of different wavelengths of light, and absorption results in the transfer of light energy to the particles in the medium.

4. How does the refractive index of a medium affect the speed of light?

The refractive index of a medium is the ratio of the speed of light in a vacuum to the speed of light in the medium. The higher the refractive index, the slower the speed of light in the medium. This is because a higher refractive index indicates a higher density of particles, which leads to more frequent interactions between light and the particles, resulting in a slower speed.

5. Can light move through all mediums?

No, light cannot move through all mediums. Some materials, such as opaque objects, do not allow light to pass through them. This is because the particles in these materials do not allow for the absorption and re-emission of light, preventing it from passing through.

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