Speed of light in materials lower than in vacuum?

  • #1

Main Question or Discussion Point

It is commonly said that the speed of light when traveling inside materials is lower than that of light in vacuum, but I don't understand how this can be true. It is the same light traveling, so how can it act differently? Does light appear to be slower in materials because it is not following a straight line and scatters off of particles and gets emits in many angles?
 

Answers and Replies

  • #2
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Does light appear to be slower in materials because it is not following a straight line and scatters off of particles and gets emits in many angles?
No. This is a common misconception.

Light is a pattern of electric and magnetic fields that induce each other. That is true both in vacuum and in matter, but the relation how electric fields induce magnetic fields and vice versa changes in matter, slowing light down.
 
  • #3
Ibix
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It is the same light traveling, so how can it act differently?
The medium is made up of charged particles which are sources for the electromagnetic field, and you can't understand light in a medium without taking those sources into account. So whether it's the "same light travelling" depends on semantics. If you track a water wave approaching the beach as it begins to break, is it the "same wave travelling"? The shape and behaviour is different nearer the shore because the environment is different, whether you see it as the same wave or not.
 
  • #4
Mister T
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It is the same light traveling, so how can it act differently?
It is the same light, but light interacts with matter, so in the absence of matter that interaction isn't present. That's the reason for the difference.
 
  • #5
Lord Jestocost
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It is commonly said that the speed of light when traveling inside materials is lower than that of light in vacuum, but I don't understand how this can be true. It is the same light traveling, so how can it act differently? Does light appear to be slower in materials because it is not following a straight line and scatters off of particles and gets emits in many angles?
What is meant at the end is the apparent speed of light. The apparent "slower speed" is the result of the superposition of two radiative electric fields, the incoming radiation and the re-radiation by the atomic electrons inside the materials, both of which travel at the normal speed of light c. On Bruce Sherwood’s homepage (https://brucesherwood.net/) you find an article “Refraction and the speed of light” dealing with this question.
 

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