Speed of Light in Materials: Explained

Click For Summary

Discussion Overview

The discussion centers on the speed of light in materials compared to its speed in a vacuum, exploring the reasons behind the perceived difference in speed. Participants examine concepts related to light's interaction with matter, including electromagnetic fields and particle interactions, while addressing common misconceptions.

Discussion Character

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

Main Points Raised

  • Some participants question how light can act differently in materials if it is the same light traveling, suggesting that scattering and emission angles might contribute to the perception of slower speed.
  • Others argue that the speed of light is affected by the interaction with charged particles in the medium, which alters the electromagnetic fields involved.
  • One participant compares the situation to a water wave approaching the shore, suggesting that the environment changes the behavior of the wave, raising questions about whether it is still the "same wave."
  • Another participant emphasizes that light interacts with matter, and this interaction is absent in a vacuum, leading to differences in speed.
  • A later reply introduces the concept of apparent speed, attributing the perceived slower speed to the superposition of incoming and re-radiated electric fields, both traveling at the speed of light in a vacuum.

Areas of Agreement / Disagreement

Participants express differing views on whether the light remains the "same" as it travels through different media, and there is no consensus on the explanation for the perceived change in speed.

Contextual Notes

Some claims depend on specific definitions of light and its behavior in various media, and there are unresolved assumptions regarding the nature of light's interaction with matter.

AndrewGRQTF
Messages
27
Reaction score
2
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?
 
Physics news on Phys.org
AndrewGRQTF said:
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.
 
  • Like
Likes   Reactions: AndrewGRQTF and Ibix
AndrewGRQTF said:
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.
 
  • Like
Likes   Reactions: nitsuj and AndrewGRQTF
AndrewGRQTF said:
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.
 
  • Like
Likes   Reactions: AndrewGRQTF and Ibix
AndrewGRQTF said:
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.
 

Similar threads

High School One-Way Speed of Light
  • · Replies 93 ·
4
Replies
93
Views
6K
High School Meaningfulness of the speed of light
  • · Replies 25 ·
Replies
25
Views
2K
Undergrad A question about the speed of light and electromagnetism
  • · Replies 51 ·
2
Replies
51
Views
5K
Undergrad One way speed of light
  • · Replies 26 ·
Replies
26
Views
975
Undergrad Why do we still say “the speed of light” instead of “c”?
  • · Replies 12 ·
Replies
12
Views
2K
Undergrad Light Speed in Dense Medium: Explained
  • · Replies 1 ·
Replies
1
Views
2K
Undergrad Conventionality of the One-Way Speed of Light
  • · Replies 6 ·
Replies
6
Views
2K
Undergrad Ships passing each other at near light speed
  • · Replies 5 ·
Replies
5
Views
571
Undergrad Can Propeller Speeds and Wave Breaks Challenge the Speed of Light?
  • · Replies 13 ·
Replies
13
Views
2K
Graduate Speed of Light for Bonnor Beam
  • · Replies 26 ·
Replies
26
Views
2K