Speed of light : missing energy

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Discussion Overview

The discussion revolves around the behavior of light as it travels through a medium such as glass, particularly focusing on the concept of speed, energy, and interactions with the medium. Participants explore theoretical implications regarding the acceleration of light and the energy dynamics involved in its propagation.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions where light acquires energy to accelerate back to the speed of light after exiting a block of glass, suggesting that it must gain energy to do so.
  • Another participant asserts that light does not accelerate, maintaining that it always travels at the speed of light, with the apparent slowing in glass being due to interactions with the medium rather than a change in the speed of the photons themselves.
  • It is proposed that the energy of light is independent of its propagation speed, implying that the energy remains constant regardless of the medium.
  • A participant references the relationship between the speed of light and the properties of the medium, indicating that changes in permittivity and permeability affect the speed of light in a medium.
  • One participant expresses uncertainty about the mechanics of light in glass, suggesting that the concept of mass and force may not apply, and that the acceleration could be instantaneous.
  • A question is raised regarding the intensity of radiation in a dielectric medium, considering how the electric field is affected and whether photons do work to orient atomic dipoles.
  • A later reply clarifies that photons do not do work in orienting dipoles, suggesting that the medium itself responds to the presence of photons without requiring work from the photons.

Areas of Agreement / Disagreement

Participants express differing views on the nature of light's speed and energy in a medium, with no consensus reached on the mechanisms involved in light's behavior as it transitions between media.

Contextual Notes

There are unresolved assumptions regarding the application of classical mechanics to light, as well as the implications of energy conservation in the context of light traveling through different media.

timharvey027
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consider light traveling through a block of glass, when it enters the block it slows down , right and is traveling at say spped of light minus X. When it emerges from the block, it is back to traveling at the speed of light. Question: At the exact point light leaves the block it must accelerate..to get to the speed of light. In order to accelerate it must get energy from somewhere...where does it get this energy?
Considering light either as a wave, stream of photon or vibrating strings it still needs more energy than it had when traveling in glass where does it come from
 
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Light never accelerates. It ALWAYS travels at the same speed.
Also, it is only the effective speed of light (for lack of a better word) that is lower in glass, the photons are still traveling at the same speed as before they entered the block; it is just that they interact with the ions/electrons in the solid but between these intereractions they are traveling at c.
 
And the energy of light doesn't depend on it's propagation speed.
 
c = [tex]\sqrt{\frac{1}{\mu\epsilon}}[/tex]
In the medium [tex]\epsilon[/tex],[tex]\mu[/tex] increases.
So,c decreases.
 
I'm no expert on the subject, and I'm not exactly sure what happens in glass...but either way light has no mass. So looking at F=ma, it becomes clear that the acceleration can easily be instantaneous.

This is probably a gross oversimplification of what happens, and it's more likely that F=ma simply doesn't apply here. Which probably also means that the simple way you're thinking about the problem also doesn't apply.

Maybe that doesn't answer the question, but at least it might help you realize that you might be thinking "in the box" :)
 
Since the presence of a dielectric reduces the net electric field, how can the intensity of radiation (I proportional to E^2) not fall?

When a photon is in a dielectric, it does work to orient the atomic dipoles in the opposite direction, right?
 
@ Mikey: Well the photon does not work to orient remember it is like induction but in this case actiona t a distance. The glass itself actually orients its particles because of the presence of the photons so it could be attracted ...so in actuality it does no work.
 

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