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Problem related to engery of light

  1. Jan 16, 2006 #1


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    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 travelling 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 !!

  2. jcsd
  3. Jan 16, 2006 #2

    Doc Al

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    Staff: Mentor

    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.
  4. Jan 16, 2006 #3
    as Doc Al said, there is no energy loss, though the momentum of the photon changes inside matter (because it is wavelength dependant).
    if you want more info about this phenomena look here:

    its pretty interesting, though it could have been aranged in a better way...
  5. Jan 17, 2006 #4

    Meir Achuz

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    Science Advisor
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    Gold Member

    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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