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Two coherent beams crossing each other's path; what will be the effect?

  1. Jun 10, 2008 #1
    A laser beam is split into two beams. After traveling some distance they cross each other's path. I was just wondering this crossing will have any effect on the beam properties or not. More basically, if two coherent beams one heading north and the other heading east cross each other at origin, what will, if any, effect on the beam properties.
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  3. Jun 10, 2008 #2
    Well if you checked them at a distance away from their meeting point they would look normal but if you put a phosophorous screen right at the point of their intersection you would see an interference pattern. (this assumes you've set it up so that both sources are at a distance such that they have the same phase at the point of intersection.)
  4. Jun 10, 2008 #3


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    Except in very unusual circumstances or with very high power beams there is no effect.
  5. Jun 10, 2008 #4


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  6. Jun 10, 2008 #5

    Andy Resnick

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    You have described a Mach-Zender interferometer.
  7. Jun 10, 2008 #6
    So, I can use a Fresnel Prism as a beam splitter?
  8. Jun 11, 2008 #7

    Andy Resnick

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    You can use anything you want as a beamsplitter, so long as it does not destroy the coherence properties of the light.
  9. Jun 17, 2008 #8
    Since photons are bosons, won't the beams just interact with each other and form a greater intensity of light?
  10. Jun 18, 2008 #9

    Andy Resnick

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    I don't understand what you are asking.
  11. Jun 18, 2008 #10
    I assume if both beams are of the same frequency they will just increase in intensity....that is all.
  12. Jun 18, 2008 #11
    I've wondered things like that myself. Looking at just one beam, The photons that are "flowing", although moving along parallel paths, I believe would resemble cars on a highway. So when it comes to constructive and destructive interference, it would depend on how the wave form of two intersecting photons line up. However such occurrences would be random within the intersection of the two beams as not every photon would actually collide with another photon. So you would get random interferences of both kinds occurring within the intersection which to a photon would be like two people running into each other within our solar system. Ultimately the reason for the increase in intensity at the intersection (seen by blowing smoke) is simply due to a higher photon density at the intersection.

    Or am I way off on this....
  13. Jun 21, 2008 #12

    Andy Resnick

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    Actually, if you are careful, it's possible to mix two different frequencies- red and green make yellow, for example.

    xArherx, photons do not travel like little bullets- that's a analogy that may be initially helpful but ultimately is internally inconsistent and leads to confusion. A single photon is of infinite extent, for example.
  14. Jun 21, 2008 #13
    I'm having trouble picturing it. I simply thought of them as the particles moving at 300,000km/s. Where each particle then carries with it, a wave form. What do you mean by "infinite extent"?
  15. Jun 22, 2008 #14

    Andy Resnick

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    A photon, strictly speaking, is an elementary excitation of the quantized electromagnetic field. In source-free space, this corresponds to monochromatic plane waves which exist for all time and space. In this sense, an individual photon is a quantum or packet of energy.

    Real wavefronts have sptially and temporally limited extent. One way to handle this is to let many electromagnetic excitations form a 'wave packet', which consists of many photons, all with slightly different frequencies (energies), thus allowing spatial and temporal localization (think Fourier transforms).

    In this way, detection of a photon is the collapse of the wavefunction.
  16. Jun 22, 2008 #15
    So the photons are considered dense packets of existing waves? Does the source-free space imply that the waves exist without a source? I'm not clear on what your saying but I'm very interested. Though I have to warn you that my current knowledge is limited, after all I did use cars on a highway as an example.lol
  17. Jun 23, 2008 #16
    Well cars on a highway collide, but photons can mix into one another. They are bosons after all and have no mass.
  18. Jun 23, 2008 #17
    Is air nonlinear enough for combining frequency? With some very high intensity, thats surely possible (even vacuum has some nonlinear properities)... but with some normal lasers - i though you need some nonlinear material for effective (visible) mixing.
  19. Jun 23, 2008 #18
    Absolutely, cars will collide. Photons can mix/constructive interference/destructive interference.

    I guess what I was thinking was that if you had two highways that intersected each other. Then cars drove through without looking, not every car will collide with another. Some of the cars will make it through the intersection unscathed. Now have many cars and the chance of a collision increases. Now make the cars really small and the chance of collisions decreases.

    I'm curious about the mass part. Photons have no rest mass. However do they have a relativistic mass? Thinking of it classically, light is affected by gravity (black holes) but only object having mass is supposed to be affected by gravity.

    So do they have a relativistic mass or do they remain massless even at relativistic speeds (meaning gravity affects them by some other reason)?
  20. Jun 23, 2008 #19


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    You have to be careful here. It is not the case, that any arbitrary photons show constructive or destructive interference, if you superpose them. They usually have to come from the same coherence volume. If any light produces interference, you would see interferences, if you just pointed two flashlights at any screen, which is not the case.
  21. Jun 23, 2008 #20
    What is meant by "same coherence volume"? Would a laser be an example of such, where the beam passes through two slots creating an interference pattern on a back screen? I've always wondered about that experiment and the shining of two flashlights is why. As you said, if you shine flashlights at the same spot there should be an interference pattern but there isn't one. How is it that the light coming from the same source shining through two slots produces the pattern while the light coming from two difference sources (kinda representing the light as it comes from each slot) doesn't? Also, I always thought that the intensity (brightness) of a light is proportional to the quantity of photons emanating from the source (or reflection). The interference experiment says that the increase and decrease in intensity is due to constructive and destructive interference. Is it one, or the other, or both?

    Although shining two flashlights onto the same spot does cause the spot to be brighter than either individual flashlight, would that mean you are simply getting a constructive interference without the destructive? Or is it simply a larger quantity of photons?

    I've always been fascinated and puzzled by light.
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