Light in Non-Vacuum: Speed of Travel?

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Discussion Overview

The discussion centers on the behavior of light and electromagnetic waves when traveling through non-vacuum media, particularly focusing on whether different frequencies of light travel at the same speed in a given medium. The scope includes theoretical aspects of light propagation, material dispersion, and references to experimental observations.

Discussion Character

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

Main Points Raised

  • One participant notes that light travels at a constant speed in a vacuum but questions the behavior of light in other media, specifically regarding the speed of electromagnetic waves with different frequencies.
  • Another participant asserts that different frequencies do not travel at the same speed in a medium, attributing this to the interaction of light waves with electronic charges in that medium, which leads to the creation of new waves.
  • A third participant introduces the concept of material dispersion, explaining that the refractive index of materials varies with frequency, which is responsible for phenomena such as the dispersion of light in glass prisms.
  • A participant mentions an article discussing the speed of light potentially changing over time and hints at frequency-dependent variations in certain situations, although they do not provide specific references at that moment.
  • Another participant presents Cauchy's Law as a mathematical relationship describing the refractive index in relation to wavelength, suggesting a formal approach to understanding light speed in materials.

Areas of Agreement / Disagreement

Participants express differing views on the speed of light in non-vacuum media, particularly regarding frequency dependence. While some points about material dispersion are acknowledged, the overall discussion remains unresolved with multiple competing views presented.

Contextual Notes

There are limitations in the discussion regarding the assumptions made about the nature of light and the specific conditions under which the speed of light may vary. The mathematical relationship provided is not fully explored, and the implications of the referenced article remain unverified.

Who May Find This Useful

This discussion may be of interest to those studying optics, physics, or materials science, as well as individuals curious about the properties of light in different media.

pi-r8
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I know light travels at a constant speed when it's in a vacuum, but what about when it isn't? That is to say, would two electromagnetic waves with different frequencies travel through the same medium at the same speed?
 
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The simple answer is no, they would not travel at the same speed - which, btw, is why we see rainbows!

It's a little more complicated than that but, basically, light waves passing through a medium interact with electronic charges in that medium. The incident electromagnetic field accelerates those charges and they re-radiate electromagnetic energy. In effect, the propagating wave causes new waves to be created as it is absorbed. In turn, the same process applies to the radiated fields.

The overall radiation field depends on the frequency of the incident wave and so the rate at which the full radiation field propagates through the medium also depends on it.
 
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The refractive index any material varies with frequency (this variation is termed material dispersion). It is this variation that allows glass prisms for example to break white light into a myriad of colours (hence Tide's refrence to rainbows).

Claude.
 
Ok, thanks. That helps a lot.
 
There was an interesting article in my New Scientist magazine this week about the speed of light changing over time, and I picked up something about the speed of light being slightly different for different frequencies in some situations. Sorry, I can't back this up online, I'll dig it out properly and post something over the weekend.

http://www.newscientist.com/search.ns?doSearch=true&articleQuery.sortOrder=1&articleQuery.queryString=%22speed+of+light%22

http://www.newscientist.com/channel/fundamentals/dn9118.html

http://www.newscientist.com/article/dn6092.html
 
Last edited by a moderator:
About speed of electromagnetic waves in material media, there is the Cauchy's Law :

n = A + B/(L^2)

where n is the refractive index : n=c/v
and L the wavelength in vacuum.
 

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