Problem related to engery of light

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Discussion Overview

The discussion revolves around the energy of light as it transitions between different media, specifically addressing the scenario of a laser beam entering a medium with higher optical density and then returning to air. Participants explore the relationship between light's speed, frequency, and energy during these transitions.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant questions where the energy comes from when a laser beam speeds up upon exiting a denser medium, suggesting a need for clarification on energy dynamics.
  • Another participant asserts that the energy of light is related to its frequency rather than its speed, indicating that frequency remains constant across media transitions.
  • A different participant notes that while there is no energy loss, the momentum of photons changes in different media, which is dependent on wavelength.
  • Another contribution emphasizes that the rate of energy passing a point remains unchanged when light enters a medium with different permittivity or permeability, suggesting that energy is conserved despite changes in speed.

Areas of Agreement / Disagreement

Participants express differing views on the relationship between light's speed and energy, with some asserting that energy is conserved while others question the implications of speed changes. The discussion does not reach a consensus on these points.

Contextual Notes

Participants have not fully resolved the implications of energy conservation in relation to changes in speed and momentum of light in different media. There are also references to specific phenomena that may require further exploration.

Who May Find This Useful

This discussion may be of interest to those studying optics, electromagnetic theory, or anyone curious about the behavior of light in various media.

uam
Hi Everybody,

I am a newbie here at Physics forums. Physics is one of my favorite subjects.

I have a doubt related to the energy of light. Any help is highly appreciated.

Consider the following situation:-

A LASER beam traveling in air at a speed of 300000 km/s enters another medium with higher optical density. So its velocity becomes (say 300000 - x km/s). Now it again enters air so its speed becomes 300000 km/s.

Now the que is:-

When it entered the medium air from glass its velocity increased by x km/s. From where did it get energy to boost up its speed by the same ?

Can anyone out there pls explain and clear my doubt. Many thanks in advance !

Regards,
uam
 
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The energy of the light is not related to its apparent speed, but to its frequency. The frequency (and thus the energy of its photons) does not change as the light goes from one medium to another.
 
as Doc Al said, there is no energy loss, though the momentum of the photon changes inside matter (because it is wavelength dependent).
if you want more info about this phenomena look here:
http://www.physics.utoronto.ca/~colloq/Talk10/Presentation10.pdf

its pretty interesting, though it could have been aranged in a better way...
 
The rate at which energy passes a given point is unchanged (not counting the energy change due to reflection) as light enters a medium
with a different epsilon or mu, so energy is conserved. The energy density in the light increases if it goes slower. The relation between energy and velocity is different for an EM wave than for a particle.
 

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