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Speed & Direction of wave

  1. Feb 6, 2012 #1
    Source cannot push or pull wave because it is not a matter. So source cannot affect speed and direction of wave.

    So why stationary observer feels diagonal path of light in light clock?
  2. jcsd
  3. Feb 6, 2012 #2
    Your conclusion is wrong. The motion of the source does affect the direction of a wave.
    Send for example light through a glass fibre, don't you think that the light will leave the fibre parallel to the fibre? Do you think that it can be different when the fibre is in motion? If you do, draw the fibre in close up and draw the light path inside the fibre. You will see that the light is guided by the fibre along with the fibre.
  4. Feb 6, 2012 #3
    Yes, light cannot leave fiber either it is moving or not. because light travels through fiber by reflecting internally.

    By vector physics if motion of the source can affect direction then it can also affect in speed if source emits wave in the same direction of motion.
  5. Feb 6, 2012 #4
    Vector algebra is used in physics; however the application depends on the theory. In relativity the speed of light is independent of the motion of the source, just like sound. Thus, try to model how the light continues its trajectory when it leaves the glass fibre at speed c in vacuum. You should see that that isn't an issue.
  6. Feb 6, 2012 #5
    If a stationary source emits some matter then the matter's path would be straight line, but if source in motion emits some matter then the matter's path would be diagonal. So the diagonal direction is propositional to speed of source in motion. The speed of source will affect both speed and direction of matter.

    But emitting wave is different scenario. Source cannot affect its speed, so why do it affect its direction? Do you think that sound source in motion can change sound's travelling path?

    Please, explain me the glass fiber stuff with some detail. I cannot get your point.

  7. Feb 6, 2012 #6
    Here you are:

    1. Light ray out of glass fibre as seen from a system in which the fibre is in rest:

    .i light
    IiI fibre

    2. Light pulse out of glass fibre as seen from a system in which the fibre is moving to the right:


    I I
    I I
    I I fibre


    .I I
    .I I
    .IiI fibre
    .I I


    ..I I
    ..I I fibre
    ..I I


    ...I I
    ...I I fibre
    ...I I


    ....I I
    ....I I
    ....I I fibre
    ....I I

    Resulting light trajectory without drawing the fibre:


    As you can see, the light emerges from the fibre under an angle as seen from the system in which the fibre is moving, such that the light continues straight out of the fibre which is moving to the right. Indeed it cannot be otherwise, just as a bullet out of a gun cannot leave the gun barrel under an angle relative to the gun. Instead the guidance of the moving fibre gives a horizontal velocity to the ray just as a moving gun barrel gives a horizontal velocity to a bullet.
  8. Feb 6, 2012 #7


    User Avatar
    Gold Member

    It is obvious if you regard light as a wave. Have a look at this animation


    The light source at the bottom is moving, and from the perspective of the initial emission the light seems to take an inclined path to the moving receiver.
  9. Feb 6, 2012 #8

    Thank you very much for detailed explanation.

    But you saying that fiber pushes wave to change its direction, if source can push the wave to change its direction then it obviously can change its speed. Because changing the direction is the property of speed of source, and the property definitely can affect the speed of wave if wave is in same direction of source.


    Suppose, that plane is on ground with running engines, then its sound waves travels in sphere. But if plane flying with super sonic speed then it creates cone of sound wave because forward waves cannot chase plane, and with upper and lower waves it creates cone. If as you say source can change wave's direction then there is a sound disk not cone.

    But as we know source speed cannot affect speed of wave, so source cannot affect wave's direction too.


    Thanks Mentz114

    But it will not showing wave in inclined path, because it transmits wave to all direction.

    Suppose, if source is stationary in your video then what would be the path of the traveling circle?

    Attached Files:

  10. Feb 6, 2012 #9


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    Staff Emeritus
    Science Advisor

    You need to be careful about using the word "obviously". A boat, moving through the water, will produce a wave in a particular direction but the speed of the water wave depends only upon the depth of the water, not the speed of the boat.
  11. Feb 6, 2012 #10


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    Gold Member

    Because from the moment the light is emitted, the receiver has moved. The light takes a longer path from the perspective of the stationary observer.

    Do you think this is not true ?
  12. Feb 7, 2012 #11


    Staff: Mentor

    mananvpanchal, do you know how a beam of light is created? Think of a spherical source and a small aperture (not so small that diffraction is an issue). Can you describe how a brief pulse of light would form a beam in such a system? Then make the source and the aperture move and describe the resulting beam using the same principles. What is the direction?
  13. Feb 7, 2012 #12
    Only moving boat can generate waves, not steady boat. So direction of boat is the direction of wave. Think about steady source which can generate one directional wave (like LASER).
    Think about how its traveling path, and move the source and think how its path. I am not telling that speed of wave is depending on speed of source. My point is neither speed nor direction is dependent of source' motion.

    Ok, Source and receiver is moving. There is source at x=0,y=0 and receiver at x=0,y=1.
    when source emits light at x=0,y=0. Why light reaches to x=1,y=1? It should reach at x=0,y=1. Who effects the light direction? the source? the speed of source? then why speed of source cannot affects speed of wave? Again I think neither speed nor direction is dependent of source's motion.

    Ok, when we think about spherical source then I think so "moving source's wave takes longer time to reach receiver". because if source and receiver is moving to right hand, then left hand part of the spherical wave reaches to receiver not front part. So it takes longer time. But what do you say about concentrated waves (like LASER) ? What would be its speed and direction?
  14. Feb 7, 2012 #13
    Suppose, we have a moving source emitting some matter

    View attachment same_dir.bmp

    If matter emitted to same direction then its speed will increase.

    View attachment rev_dir.bmp

    If matter emitted in reverse direction then its speed will decreases.

    View attachment per_dir.bmp

    If matter emitted in perpendicular direction then its speed increases and direction changed.

    So, no matter what source's motion can affect speed of matter and in some case direction.

    But what I am thinking is if source emits wave then source motion cannot affect speed and direction.

    But you are saying if source's motion can affect wave's direction but cannot speed.
  15. Feb 7, 2012 #14
    You refer to a source that goes faster than the wave; no such thing is possible with light. And it is "obvious" (assumed, reasoned and not disproved by experiments) to people who work in the field that a moving source cannot affect the speed of a wave that propagates away from it at a distance. A sound wave is the propagation of vibration of gas molecules; and at least at a distance, those molecules are not in any way affected by the motion of the air plane.
    I'm sorry, repeating an erroneous line of reasoning that you already know to be wrong is counterproductive. Look again at my drawing: the velocity to the right is imposed by the glass fibre, however the velocity "up" is not imposed by the glass fibre. The source can only influence the direction of a wave, not its speed.

    Here's another way to look at it: you can direct a wave in any direction that you want. Then, after you have released the wave, it propagates independently from you; the speed of a wave is independent of that of the source as it instead depends on the medium that propagates it. For sound, that medium is the air; for light we can only infer that there is something which we just call "space". Many people think that there is nothing, but that makes it hard to explain the existence of the properties of space which general relativity describes. Therefore Einstein explained it this way.
    Last edited: Feb 7, 2012
  16. Feb 7, 2012 #15
    If source cannot affect speed of wave then how it can affect direction of propagation of wave?

    Forget about light just now because its velocity is constant.
    UP speed is not imposed by source, but RIGHT speed is imposed by source.
    Do you agree that resulting speed is = (up + right)? (In case of wave except light)

    Again I am clearing my point "I am not telling that moving source can affect speed of wave. I am telling that moving source can neither affect speed nor direction."
  17. Feb 7, 2012 #16
    You can shine a torch in any direction; and you can also point a loudspeaker in any direction. However, the way you orient your torch or your loudspeaker has no effect on the speed of propagation - there is no reason to think so. :wink:
    For a wave the resulting speed can only be c - for all waves, thus including light waves. As you know c, you can easily determine with vector analysis what the resulting upward velocity will be.
    Your point - which should have been a question - is erroneous as it is in conflict with both experiment and theory. This forum is meant to explain this "mainstream"model of light propagation; it's useless and even against the forum rules to try to push other theories that are in conflict with the experimental evidence. Sorry.
  18. Feb 7, 2012 #17
    Ok, that is my lake of knowledge that I don't know what is the experimental evidence that tell me that when I propagate wave to the north and the source is moving to east then the wave propagates to the north-east. Please, let me know about the experiment.

    One moving light clock with LASER. Please, explain me why there is diagonal path of the laser beam?
  19. Feb 7, 2012 #18


    Staff: Mentor

    And you also understand that it goes diagonally, correct?

    Here I appeal to the principle of relativity.

    Suppose you have a spherical source and aperture like described above and you have a laser, both at rest. You align them next to each other and mark their light beam's positions on some screen, also at rest.

    Now, in a frame where the sources and screen are moving, the principle of relativity requires that the beams mark the same spots as previously. Since we know that the aperture-based beam goes diagonally and marks its same spot, then we also know that the laser must go diagonally to mark its same spot.
  20. Feb 7, 2012 #19


    Staff: Mentor

    But you have already agreed that it can for an emitter based on a spherical source enclosed with an aperture.
  21. Feb 7, 2012 #20
    Experiments with lasers have been done in an effort to measure a deviation from "straight out" relative to the laser, but no significant result was found.

    It's exactly as with the glass fibre: a laser is made of mirrors, and the mirrors direct the light to be emitted parallel to the laser cavity, perpendicular to its mirrors. So, it's again the same drawing as in post #6. The light cannot (and wont!) bend in another direction when leaving the cavity. :tongue:

    Here is the resultant trajectory again (mostly inside the laser, but continuing outside it):

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