Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

A question about Light

  1. Apr 1, 2004 #1
    As this is my first posting please excuse me if my approach is wrong.

    The question I wish to ask was posted at another forum and I thought it a valid one.
    When experimenting with the idea of capturing light I have tried doing so using crystals and found that the only result was that the crystal "glowed" but the interesting thing was that when the laser light was switched off the crystal fell dark immediately.

    If as the above question suggests the beam of light is travelling and is constantly being refected within the crystal then I find the immediate extiguishing of the light in the crystal puzzling.

    Can any one help me with this as I find it a little confusing?
    Last edited by a moderator: Apr 1, 2004
  2. jcsd
  3. Apr 1, 2004 #2


    User Avatar
    Staff Emeritus
    Gold Member

    The laser light will not reflect perfectly from the boundaries. Some energy will be absorbed by the material. After several reflections, the beam will be much attenuated. Then, if you think about the speed of light and the size of your "light-container", you can get a feeling of how much time is needed for the glow to fade away. It is not immediate, but too fast for your eyes to catch.
  4. Apr 2, 2004 #3
    If you did theoretically have a perfect mirror box in a vacuum, so there was nothing inhibiting the light from ongoing reflections (highly improbable), and you could fire a laser in, the box would always be dark. Since the light is uninhibited in this box, I take that to mean there will be no energy loss in the box, there can be no light leaving the box. If no light leaves the box there will be no light for your eyes to receive, hence always dark.

    I’m sorry if misinterpreted the question.
  5. Apr 2, 2004 #4
    The image of the laser light would be seen in the mirrors I would suggest. ( have tried this at home)

    A web page link ( experiment schematic) may help in understanding the question a little better ( also posted at another forum)
    Inverse light speed experiment
  6. Apr 2, 2004 #5


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    The trouble with this experiment is the same that plagues every one way speed of light effort. You need to know the time between the light shutting off at point A and then at point B. How do you communicate the fact to point B that the light has been shut off and the clock should start counting? These to points are separated by a physical distance, the problem of synchronizing clocks the clocks. You can run a cable between the 2 points but you still have a signal delay to deal with, how do you measure the signal delay? Do you ASSUME it occurs at the speed of light? Any measurement of it will run into the same problem you are attempting to solve. If you attempt to synchronize to clocks, the degree of accuracy required will be effected by the mere act of moving one while the other remains stationary. Those are problems involved with getting a meaningful time measurement.

    To the best of my knowledges there is no such thing as a 100% reflective mirror. Every real world mirror has losses of some sort. Light is either adsorbed, transmitted or reflected. Only the transmitted portion can reach the final detector, each reflection suffers the same loss. Assume the you have a mirror which is able to transmit 99% of the incident light since each reflection passes 99% of the light the amount of light after n reflections is .99n, after 100 reflections you are down to ~35% of your original light. Combine this with the divergence of your laser beam (all real laser beams have a divergence angle) You will find very real restrictions on the distance you can get.
  7. Apr 2, 2004 #6
    wow, Integral, thanks for that......your analysis is terrific....how ever if we pose the hypothetical and allow for perfect reflection and the beam maintaining integrity over a distance of 10 light seconds what would we see?

    Would there be a 10 second delay before the beam extinguished in full after the source was switched off or interrupted(blocked)?

    I think that according to light theory that this would be the case.
    The clock problem I think could be overcome by placing angular mirrors appropriately so that the source and end reflection are very close. (side by side) and possibly a rotating mirror could be used to detect an angle for measuring but I'm not sure....
    The main thrust of the experiment is to confirm that light travels by applying "from" basis rather than a "to" basis if you catch my drift.

    Possibly you can think of a better way to achieve this evidence??
    Last edited by a moderator: Apr 2, 2004
  8. Apr 2, 2004 #7


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    I am not sure what you mean by "what would we see".... The light is on, the light is off.

    One small factor I forgot about in the first analysis. At each reflection, the photons will be ADSORBED by the mirror, after a very small time interval a photon will be remitted. This is called Quantum Mechanics, it is every bit as inescapable as gravity. This effect will completly swamp any possible speed of light measurement you could hope for. The only way to get a meaningful one way speed of light measurement is to measure the speed of an undeflected light beam in free space.

    Sorry, no meaningful numbers can possibly come from such an experiment as you have proposed.
    No, I do not catch your drift. What does this mean?
  9. Apr 2, 2004 #8
    Please correct me if I am wrong....
    At present light is only measured by it's end effect, at the end of it's journey using a continuous beam of light.

    I propose that an experiment be developed to prove that a photon actually travells and not just the time measured of the difference at the end of it's journey. Go an extra step and prove that when the source is switched off the last photons to be sent on their way actually do go all the way. Thus measuring the departure point and the destination point.

    As you have gathered I already have an idea that this would prove that light in fact doesn't travel and that what we are measuring is in fact a lightspeed change in intensity in continuum "source to destination"

    This would be just a small step in clarifying the distinction between intensity and speed regarding distances.

    Ultimately I want to rule out or in that light may be an "Instantaneous gravitational resonance" ....that light is a sympathetic effect of reflected energy and gravity being the mirror.

    This concept of course belongs in theory development. The experiment though belongs I feel here in General physics.

    I might add this is not the direction I intended this thread to go.....but have done some thinking since posting it and well...here we are

    Gravitational harmonics and resonances are an old idea from many years ago and it now seems appropriate to mention them.
    Last edited by a moderator: Apr 2, 2004
  10. Apr 2, 2004 #9


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    If I understand what you are saying, yes this does belong in Theory Development and, yes I believe you are wrong.

    I notice that you did not addess the issue with your experiment?
  11. Apr 2, 2004 #10
    The issue is simply about proving light travels, the suggested experiment was one way but as you have said the method suggested is impracticle.

    Can you offer a better way to prove beyond doubt that light travels?

    What is really curious about all this is that this approach seems not to have been tried.

    Integral with all due respect just to be sure we are not at cross purposes what do you think is the issue I haven't addressed with this experiment?
    Last edited by a moderator: Apr 2, 2004
  12. Apr 2, 2004 #11
    or another question would be How long does it take a reflector to generate the reflection and what conditions apply to that speed?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook