Why does light appear to slow down in different mediums?

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Discussion Overview

The discussion revolves around the behavior of light as it travels through different media, specifically addressing whether light slows down in these media or if the observed effects are due to delays in absorption and re-emission by atoms. The conversation touches on theoretical concepts, interpretations of refraction, and the implications of relativity.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • Some participants assert that the speed of light is always constant at "c" regardless of the medium, suggesting that any perceived slowing is due to delays in absorption and re-emission by atoms.
  • Others question whether this delay can be considered as slowing down, indicating a potential ambiguity in the definition of "slow down."
  • There is a discussion about the relationship between light's speed, wavelength, and frequency in different media, with some participants proposing that changes in wavelength are a result of changes in the medium's density.
  • One participant raises a question about the coherence of laser light in refractive media, seeking clarification on whether the wavelength is genuinely shorter in higher indices of refraction or if it is merely a calculation artifact.
  • Another participant introduces the concept of momentum conservation during the absorption and re-emission process, suggesting that this might influence the direction of emitted photons.

Areas of Agreement / Disagreement

Participants express differing views on whether light truly slows down in a medium or if the observed effects are merely due to delays. There is no consensus on the definitions of "light" and "slow down," leading to ongoing debate and exploration of the topic.

Contextual Notes

The discussion highlights the complexity of interpreting light behavior in different media, with participants acknowledging various assumptions and definitions that influence their arguments. The nuances of refraction and the interaction of light with matter remain unresolved.

nautica
Speed of Light?

Would someone clear up the speed of light? Is it dependent on the medium in which it travels, or not? Depending on what you read. It says that light always travels at "c". Or it can be slowed down "refracted" depending on the medium.

Nautica
 
Science news on Phys.org
The speed of light is always a constant irrespective of the frame of reference or medium. The apparent slowing down of light due to refraction is due to the fact that the atoms inside the medium absorb the light and then emit the light after some time delay. This time delay in the transmission of light across the medium gives a feeling that light has been slowed down by the medium!. So as you see, light never travels wit a speed < or > c. It infact travels at c irrespective of where it is headed for, or who makes the measurement, or the medium in which it is traveling in.

Understood?


Sridhar
 
so it does not in fact slow down at all? only delayed? then wud tht not be slowing down?
 
Take your choice. The crucial point of relativity is that the speed of light in vacuum is constant irrespective of the frame of reference.

From a macro point of view, light is slowed down by passing through matter. From a micro point of view, light travels at c from atom to atom and what we perceive as slowing down is the slight delay when light is absorbed and then retransmitted by atoms.
 
thanks for that.
but if someone asks me does light slow down in a different medium i say...
 
but if someone asks me does light slow down in a different i say...
... you say that "Yes light is slowed down by the medium as perceived by us at the macro level - However what actually happens is like ...(the previous explanation given that the apparent slowing of light due to delayed transmission of light through the medium) "

Sridhar
 
thanks people
 
Originally posted by FUNKER
... if someone asks me does light slow down in a different medium i say...
Ask that person what they mean by "light."
 
And, of course, require that they define "slow down"!
 
  • #10
The reason refraction made sense to me was that as light entered a denser medium its speed decreased and due to its frequency remaining the same the wave length also decreased, balancing the equation c=f.<lambda>. This change in wavelength was then responsible for the change in direction:

refraction.gif


Why should refraction exist if the delay is due to light is being transmitted from atom to atom? If this were the case I would expect light to emerge from a medium on a random vector.
 
  • #11
Originally posted by MisterBig
Why should refraction exist if the delay is due to light is being transmitted from atom to atom? If this were the case I would expect light to emerge from a medium on a random vector.
That sounds like a good question to me. My guess would be that, since momentum is conserved, even when a photon is absorbed, the momentum still exists, so there is some preference to emit a photon with that momentum. That's just a guess, though.

Now I got to thinking, about the wavelength. Is the wavelength truly shorter in a higher index of refraction? Or do we just use the shorter wavelength as a pseudo wavelength to do calculations? I guess the first question I should ask: can the light in a refractive medium be coherrent? I've seen laser light in air, but I'm talking about significant refraction. Is laser light coherrent in water?
 
  • #12
Originally posted by FUNKER
so it does not in fact slow down at all? only delayed? then wud tht not be slowing down?

Look at it this way: A photon hits a flat piece of glass head on. The photon is absorbed by a particular atom, then after a very brief time, is re-emmited. It is directed to the next atom which does the same thing and so on until it exits the glass.
The photon emmited and re-emmited always is going the speed of light. But, the CUMULATIVE time delay of all the atoms involved in absorption and re-emmission accounts for the apparent, but NOT REAL, slowing down of the photon itself.
 

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