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Speed of light through a prism

  1. May 10, 2010 #1
    When light is observed going through a prism the speed changes and so does the wavelength but the frequency remains the same.
    That is why we see the different colours.
    Once the light exits the prism we still observe the colours of the spectrum produced.
    So is the coloured light observed exiting the prism travelling at c or are the constituant parts all still travelling at different speeds and any colours we observe from different objects have different wavelenths and speeds but the same frequency.
     
    Last edited: May 10, 2010
  2. jcsd
  3. May 10, 2010 #2
    Your latter choice is correct; the different colors are still light, just separated by wavelength/frequency, and they would go back to the speed of light once they left the glass.
     
  4. May 11, 2010 #3
    So where does the extra energy come from to go back to the speed of light and if it was never lost(say it took a longer path through the prism) why are they still different colours.How come the coloured light does not reform into a single coloured light when exiting the prism.
     
  5. May 12, 2010 #4
    The energy does not change. The reason is that in the prism the light has the same frequency (thus the same energy since E=h*f), but the wavelength changes.
    See here.

    The different colors are seperated due to the geometry of the prism and refraction. Refraction is the phenomenon that light is bent between two media (such as air and glass). How much the light is bent also depends on the frequency of light. That is why red light is refracted differently than blue light (see refractive index).
     
  6. May 12, 2010 #5
    So if red light is refracted differently than blue it must have a slightly different frequency and be traveling at a different speed.
    Presumably inside the prism and when it exits.
     
  7. May 12, 2010 #6
    Of course they have different frequencies but not different speeds.
     
  8. May 12, 2010 #7

    Doc Al

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    Yes, the different frequencies of light have different speeds inside the material of the prism.
    Only inside the prism. Once they exit, all frequencies have pretty much the same speed.
     
  9. May 12, 2010 #8
    What is the effect that makes the different frequencies go from a non uniform speed inside the prism to a uniform speed when the light exits.
     
  10. May 12, 2010 #9

    Doc Al

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    The dependence of speed (or index of refraction) on frequency is called chromatic dispersion. See: http://en.wikipedia.org/wiki/Dispersion_(optics [Broken])
     
    Last edited by a moderator: May 4, 2017
  11. May 13, 2010 #10
    So from what is stated it's the material through which the light travells ie. the glass, that causes the different colours to have different speeds.
    So when they exit the speed is pretty much the same.
    How is it possible to conclude that the speeds are the same when the colours we observe have to travell through the lens of our eye once they exit the prism, won't the lenses in our eyes act in a similar way to glass and refract the light and any experiment we do will be dependent on our sight.
     
  12. May 14, 2010 #11
    If I understand this correctly the separation of colors is due to frequency changes caused by the refraction going on inside the prism. As for when light is traveling inside the prism it is still traveling at speed c. It is getting bent and absorbed from the atoms and molecules inside the glass the re-emitted. Therefore this leads to light appearing to travel slower than c. But from atom to atom or molecule to molecule it still travels at c.
     
  13. May 14, 2010 #12

    Doc Al

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    That's right.
    Right.
    Yes, it's true that light will be refracted by the material of our eyes (which is what allows us to form images on the retina). I don't know the dispersive characteristics of our eyes, but I'm sure there's some. I don't see what that has to do with measuring the speed of light, though.
     
  14. May 14, 2010 #13

    Doc Al

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    The refraction is due to wavelength/phase speed changes as the light enters the medium, not to frequency changes. Each different frequency gets a slight different amount of refraction, thus allowing the colors to be separated.
    The phase speed of the light wave is reduced in the material, but the speed of actual photons is still c.
     
  15. May 14, 2010 #14
    As far as I am aware non of the measurements take into account the refractive index of the material in your eyes.
    So the observed speed and actual speed are slightly different.
     
  16. May 14, 2010 #15

    Doc Al

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    Give me an example of a speed of light measurement where the refractive index of your eyeball plays a role. Realize that any realistic measurement will involve instrumentation, not someone with a stopwatch waiting for a light flash to hit his eye.
     
  17. May 14, 2010 #16

    Cleonis

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    I wear pretty strong glasses (-8.5), and the dispersion of light in the lenses of my glasses is very noticable. Anyway, my glasses are auxillary; I wear contact lenses

    In the case of my glasses: in the center of the lenses the outside and the inside of the lense are almost parallel. It is towards the rim of the lenses that the lens is acting more and more as a prism. If I look straight ahead I don't see dispersion effects. If I look sideways, looking through a prism-like part of the lens, then dispersion is strong.

    Now, in case of the eye we are in fact using pretty much only the center of the eye-lens. A small patch of the retina, called the fovea, is doing all the work. The fovea is densely packed with very small retinal cells. Further from the fovea the retinal cells are larger and less densely packed.

    The fovea is the only part of the retina that provides sharp vision. We get the impression that we see sharply in the entire field of view because the very instant that some item catches our attention our eyes home in on that item, bringing the image of that item in the fovea.

    Our peripheral vision is good at noticing motion, and change in general, but peripheral vision is not sharp at all. Try it. Take a text, focus on a particular word in that text, and then try to read adjacent words.
    Initially your eyes will simply flick over to those adjacent words, but if you can manage to keep your gaze fixed you'll find that you're not able to read the adjacent words. Outside of the fovea your vision is far inferior.

    I think that is why we don't see dispersion effects that arise from dispersion in the lens of the eye. Presumably the dispersion effects do occur, but the peripheral vision can't discern them.
     
  18. May 14, 2010 #17
    All of them indirectly.
    Every instrument ever made has evolved from others that were calibrated at first using your eyes.Somehowe I don't think it occured to take into account the refraction within your eyes.
    Maybe we should try the latest methods and see if there is any role played.
    What we need is a good description.
     
  19. May 14, 2010 #18

    Doc Al

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    A digital meter outputs "10.3" (for example). Do you think that needs to be corrected due to the refraction in your eyes? (Of course, one may well need glasses to read the numbers. Is that what you mean? :smile:)
     
  20. May 14, 2010 #19
    Not 100% sure but I can imagine that somewhere along the lines of it's manufacture and it's calibration there would be inherent flaws from some previouse model which would have been done by eye.So the meters output could have hereditory mistakes built into them.
    Even the most modern Laser Interferometers will have to be set up using your eyes.
    So if we suppose that all the flaws have been eliminated and the actual reading is 10.3 for a given distance we are still left with the conundrum is it actual or observed distance.
    The meter might be right but my eyes tell a different story.:wink:
     
    Last edited: May 14, 2010
  21. May 15, 2010 #20
    Dispersion effects happen on the journey from the lens to the fovea as well as through the lens, the light after going through the lens travels through the vitreous humour.
    Which has a refractive index of 1.336.
     
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