Is the speed of light a constant

Click For Summary
SUMMARY

The speed of light in a vacuum, denoted as 'c', is a constant approximately equal to 3x10^8 m/s and cannot exceed this value. While individual photons always travel at 'c', their effective speed can appear slower in various media due to interactions with atoms, which affects the group velocity of light pulses. This phenomenon is often misunderstood, leading to confusion between the speed of individual photons and the speed of light as it propagates through different materials. The discussions clarify that while the speed of light is constant in a vacuum, it is the interactions in a medium that result in a slower average speed of light pulses.

PREREQUISITES
  • Understanding of the concept of the speed of light in a vacuum (c)
  • Basic knowledge of light propagation and refraction
  • Familiarity with photon behavior in different media
  • Awareness of quantum electrodynamics (QED) principles
NEXT STEPS
  • Research the principles of light refraction and the refractive index of various materials
  • Study quantum electrodynamics (QED) to understand photon interactions with matter
  • Explore the concept of group velocity versus phase velocity in wave mechanics
  • Investigate historical theories regarding the speed of light and time travel concepts
USEFUL FOR

Physicists, students of optics, educators in physics, and anyone interested in the fundamental properties of light and its behavior in different media.

Physics news on Phys.org
  • #32
Infact what count is "signal velocity" which however is not always very easy to define.
 
  • #33
lightarrow said:
Infact what count is "signal velocity" which however is not always very easy to define.

Yes.

In a medium with normal dispersion and n > 1, most of the energy propagates at a speed equal to the group velocity (less than c), but there is an initial small signal, called a precursor, that propagates almost at c. So, if one has sensitive detectors, signal velocity in any medium is very close to c. Sommerfeld was one of the first people to work this out, and I first learned about this in Jackson.
 

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 I want a good reference to a discussion of the meaning of "the speed of light is constant"
  • · Replies 60 ·
3
Replies
60
Views
5K
Undergrad A question about the speed of light and electromagnetism
  • · Replies 51 ·
2
Replies
51
Views
4K
High School Why is the constant speed of light so unique?
  • · Replies 19 ·
Replies
19
Views
1K
High School Why Do Events Freeze For Me at the Speed of Light?
  • · Replies 4 ·
Replies
4
Views
676
High School Simplest symmetrical time dilation question
  • · Replies 16 ·
Replies
16
Views
2K
High School Constant Speed of Light: Low Speed Explained
  • · Replies 18 ·
Replies
18
Views
2K
High School Questions about the Speed of Light, Time, etc.
  • · Replies 74 ·
3
Replies
74
Views
5K
Undergrad Time dilation and the human body
  • · Replies 11 ·
Replies
11
Views
2K