Why does the speed of light change in different media?

  • #1
ColdheartedGod
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In relativity, the speed of light in vacuum is a universal constant. Also, it has wave-particle duality. So if the speed of light slows down in a different media other than vacuum, what exactly is slowing down?
Macroscopically speaking, the speed of light does slow down. What about in the microscopic points of view?If as particles, photons are much smaller than electrons and nuclei, so basically it's moving in vacuum with frequent bouncing. Can we say we measure the speed in other media slower than that in vacuum is because photons don't travel alone a straight line as in vacuum but twisty lines due to the bouncing with other particles?
My teacher taught me, if we treat light as waves, when the light enter a new optical dense media, the light rays will collide with particles, lose energy and slows down. But it doesn't make sense when they reenter a media with smaller refractive index, the light rays speed up.
Personally, I cannot understand why a universal constant which plays such a huge role in relativity would be influenced easily by media. With the vacuum fluctuation, we cannot have a real "vacuum", thus the speed cannot be precisely measured. There must be something special about it which make up the speed limit of our universe!!!
 

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  • #2
sophiecentaur
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My teacher taught me, if we treat light as waves, when the light enter a new optical dense media, the light rays will collide with particles, lose energy and slows down.
This was wrong in many ways. The photons that are not absorbed by the medium (some of them will be) do not have their Energy (=frequency) changed. The power of the light beam will be attenuated; the interaction of the unattenuated waves and the substance introduces a lag, which means there is a delay on the way through. That means that the speed of the wave will be measured as slower. It is the wavelength but not the frequency that changes. Once the ray emerges into vacuum (or near vacuum) the wavelength goes back up so that fλ is back to c. (No acceleration is involved - it's not Mechanics)
With the vacuum fluctuation, we cannot have a real "vacuum", thus the speed cannot be precisely measured. There must be something special about it which make up the speed limit of our universe!!!
From your first quote to to this quote requires a massive jump in understanding. "Vacuum Fluctuations" are a very sophisticated concept.
c is so fundamental that you can't really hang it on the measurement method; it's more the other way round. There's more to c than just a speed limit. Wherever you are, you will measure c as the same. (There's no other speed like that; all other speeds depend on the observer's situation)
 
  • #3
PeroK
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In relativity, the speed of light in vacuum is a universal constant. Also, it has wave-particle duality. So if the speed of light slows down in a different media other than vacuum, what exactly is slowing down?
Macroscopically speaking, the speed of light does slow down. What about in the microscopic points of view?If as particles, photons are much smaller than electrons and nuclei, so basically it's moving in vacuum with frequent bouncing. Can we say we measure the speed in other media slower than that in vacuum is because photons don't travel alone a straight line as in vacuum but twisty lines due to the bouncing with other particles?
My teacher taught me, if we treat light as waves, when the light enter a new optical dense media, the light rays will collide with particles, lose energy and slows down. But it doesn't make sense when they reenter a media with smaller refractive index, the light rays speed up.
Personally, I cannot understand why a universal constant which plays such a huge role in relativity would be influenced easily by media. With the vacuum fluctuation, we cannot have a real "vacuum", thus the speed cannot be precisely measured. There must be something special about it which make up the speed limit of our universe!!!

This question was asked recently. See:

https://www.physicsforums.com/threa...ight-smaller-in-a-medium.977743/#post-6236852
 
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  • #5
PeroK
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Personally, I cannot understand why a universal constant which plays such a huge role in relativity would be influenced easily by media.

Note that this is entirely the wrong way to think about physics. This is almost like a political argument! Light is electromagnetic radiation and a vacuum is a vacuum. Hence the speed of light in vacuum is constant. A medium, however, has charged particles that interact with EM radiation. There is no reason, therefore, to presume that the speed of light in a medium is the same as the speed of light in a vacuum.

Note, however, that the speed of light in vacuum is also "invariant", meaning the same in all inertial reference frames. That is the key property that underpins relativity and disrupts the notions of classical physics.
 
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