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Light as a wave?

  1. May 18, 2010 #1

    Fuz

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    Hey guys,

    I don't see how light can be a wave. If it is, then that means the velocity of light would not be constant. It would have to speed up when blue shifted because the wavelength is shorter. When the wavelength is short, it would have to travel faster up and down to make up for the horizontal speed.

    I'm thinking of light as literally moving like a ripple. A broader question would be, what is a wave? Does light move in an up and down motion like a ripple or is it actually linear?

    An idea my friend and I came up with is that light isn't a wave, but a pulse. The shorter the wavelength, the faster the light pulses. Are we right, or is light just a constant stream?

    BTW I'm talking about the physical aspect of light moving through a vacuum (the actual lights motion).
     
  2. jcsd
  3. May 18, 2010 #2
    Light is a flux of photons that are quantum mechanical particles, and, hence, do not obey the ordinary laws of mechanics of bodies.
     
  4. May 18, 2010 #3
    Don't confuse the longitudinal speed with the field intensity in say the vertical direction. It's not a velocity but an intensity.

    Light is not a constant stream or a pulse. It is a pair of vector wave fields (electric and magnetic).
     
  5. May 19, 2010 #4

    diazona

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    Homework Helper

    Sounds like you have some misunderstanding of how waves work. Why would you say that a change in the wavelength should change the speed of the wave? And what's this about up-and-down motion?
    Physically speaking, a wave is any disturbance that propagates through a medium. For example, in a sound wave, the disturbance could start as a region where the pressure is higher than in the surrounding air. That high pressure air compresses the air in a shell around it, which in turn compresses the air in another shell around that shell, and so on. So the high pressure spreads outward. Here's a nice animation (well, actually a Java applet) to show how that works:
    http://illuminations.nctm.org/ActivityDetail.aspx?id=37
    Similarly, if you throw a rock in a pond you get a disturbance (a ripple) that travels outward. For electromagnetic waves, the electromagnetic field is the medium.
    Up and down depends entirely on which way you're standing :wink: but let me see if I can rephrase your question: "Do light waves move perpendicular to the direction in which they propagate like ripples do?" If that's what you're trying to ask, the answer is no. In a light wave there is nothing that actually moves "sideways" (perpendicular to the direction the wave is traveling), or in any direction. Going back to the definition I gave above, the disturbance is a region of high-intensity electric and magnetic fields. It propagates along the same way as a sound wave. But there is nothing that actually shifts position as the wave travels; it's all in the intensity of the electromagnetic fields.

    Here's a good animation of a light wave:
    http://www.molphys.leidenuniv.nl/monos/smo/index.html?basics/light_anim.htm [Broken]
    It uses arrows because the electric and magnetic fields are vector fields - at every point in space, each field has both a magnitude and a direction. Each arrow represents the value of the field at the point where the base of the arrow is located.
    I don't know... you might be thinking of the right idea, but I can't really tell from what you're saying here.
     
    Last edited by a moderator: May 4, 2017
  6. May 19, 2010 #5

    Dale

    Staff: Mentor

    You are thinking about a wave on a string and misunderstanding what is meant by the speed of a wave.

    If we consider two waves on a string, one of a higher frequency than the other, but both of identical amplitude. The particles of the string will be moving faster on average in the one with the higher frequency, but that is not what is meant by the speed of a wave. The speed of the wave is not the speed of the particles in the string but length of the string divided by the amount of time that the wave takes to travel across the string. This speed would be the same for both waves.
     
  7. May 21, 2010 #6

    Fuz

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    Thanks for the helpful replies :smile:

    For the record, I actually never thought that light moved in a ripple like path, I was mainly wondering what a wave actually was and why physicists even call light a wave (or do they?).

    Another thing I'd like to ask is whats the difference between a photon and light? Do either have any physical dimension?
     
  8. May 21, 2010 #7

    Dale

    Staff: Mentor

    Physicists call light a wave because it exhibits behaviors of waves such as reflection, refraction, and diffraction.
     
  9. May 22, 2010 #8

    diazona

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    That kind of depends on what you mean by "physical dimension." Does sound have any physical dimension? Well, it does have a wavelength, but it's not like sound is composed of solid particles which have some specific size. Same with light.

    Technically, a photon is a quantum of the electromagnetic field. That is, the energy in an electromagnetic wave comes in discrete units (there's no intuitive reason for this, it's just an observation), and sometimes it's convenient for us to think about each unit corresponding to a particle, which is the photon.
     
  10. May 22, 2010 #9

    Fuz

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    Sorry, I should have been clearer. I was basically asking whether photons (or light) take up space (have any volume), like an atom for example.
     
  11. May 22, 2010 #10

    DaveC426913

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    Gold Member

    ...and frequency and wavelength and constructive/destructive interference. (I mention these because, unlike the ones you mention, these are peculiar to waves).
     
  12. May 22, 2010 #11
    I usually think of it as, its a wave because its a solution to the wave equation. If its a solution, its a wave - if its not a solution, its not a wave. Now I wonder, do all solutions of the wave equation exhibit reflection, refraction and diffraction? I wouldn't be surprised, but I dont know.
     
  13. May 23, 2010 #12

    Fuz

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    What is wavelength actually measuring? I know that the shorter the length, the higher the energy (and bluer the light), but what is the length (meters) in wavelength tell us about the radiation?

    I understand that it's telling us the distance between the peaks of the wave, but does that distance tell us anything else?

    Thanks for everything,
    Fuz
     
  14. May 23, 2010 #13

    Dale

    Staff: Mentor

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