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The concept of waves

  1. Jul 31, 2011 #1
    Hello World,
    These days, I'm trying to introduce the concept of waves into my stupid mind but It seems to be Hopeless..Whatever..
    What are waves really?Can we see waves by any means in the world ?or they just do disturbance to the medium particles?

    Is there a real hope to understand waves?

  2. jcsd
  3. Jul 31, 2011 #2


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    Have you ever dropped a pebble into a pond? Those ringey things on the surface traveling away from where the pebble went in are waves.
  4. Jul 31, 2011 #3
    The wave-like properties of light and other elementary particles can be observed through the double-slit experiment. Youtube search double-slit experiment. Unfortunately, Physicists even today cannot agree as to whether photons and other quantum particles create the wave, ride a preexisting wave, or simply just follow a wave-like path since that is the nature of all movement. Physics today is a mess of conflicting theories and paradoxes, and it all comes back to wave-particle duality. Once you get into physics and you hear things like "quantum weirdness"--you know they're hopelessly lost, if they're just saying "man thats weird" out loud.
  5. Aug 1, 2011 #4
    so we can see waves according to you?

    my question is quite simple and I think i can't get the answer that satisfies my eagerness

    Last edited by a moderator: Aug 24, 2011
  6. Aug 1, 2011 #5
  7. Aug 1, 2011 #6


    Staff: Mentor

    In one clear and unambiguous sentence, what is your simple question?
  8. Aug 1, 2011 #7
    Well,sorry,It's not one question..it's a combination of many questions , i can't introduce the concept of a wave in to my mind and I'm suffering a lot ..
    First of all :Can we see waves by any means in the world ?or they just do disturbance to the medium particles?
    What does this represent?Don't tell me a wave..Does this represent the vibrations of particles of the medium? If so then what is a wave?Can this wave be seen??

    Thanks in advance
  9. Aug 1, 2011 #8


    Staff: Mentor

    Then it is a little unfair for you to respond to Renaatier as you did.

    In physics, a good definition of a wave is any phenomenon which behaves according to the wave equation:

    There are many different systems that behave according to the wave equation, so yes, we see waves by many means in the world. Some waves are disturbances to some medium of particles, but others are disturbances to fields. It doesn't really matter what the function (u in the Wikipedia article) represents, only how it behaves.
  10. Aug 1, 2011 #9
    hmmm so we see water waves?these circles are called water waves then?
  11. Aug 1, 2011 #10
    but the wave equation is very complicated for me and we don't study these things at school>>are there any simpler ways?
  12. Aug 1, 2011 #11
    It depends on the type of wave. In water, the shape of the wave shows where the water is physically higher. That's the one you're used to.

    With light, it represents the strength of the electric field at that point. In quantum mechanics, it represents where something is likely to be found. Waves can mean many things depending on the context!
  13. Aug 1, 2011 #12


    Staff: Mentor

    There are certainly simpler ways to understand specific kinds of waves, but to understand the general underlying phenomenon I think requires the math. Is there a specific kind of wave that you are interested in for simplicity's sake? If so, then we can probably discuss that specific kind of wave without a lot of equations.
  14. Aug 1, 2011 #13
    ok,let's talk about water waves and sound waves
  15. Aug 1, 2011 #14


    Staff: Mentor

    Let's start with sound waves. In a sound wave we are looking at small spatial and temporal deviations in the air pressure and velocity from the "bulk" pressure and velocity.

    If you consider a small chunk of air then if the pressure on the left is greater than the pressure on the right there will be a net force on the chunk which will cause it to accelerate to the right. In doing so it will get closer to the neighboring chunk on the right and further away from the neighboring chunk on the left. This will cause the pressure on the right to increase and the pressure on the left to decrease, bringing both back to equilibrium.

    However, by the time that the pressure equalizes the chunk of air will have gained some velocity to the right, and due to its natural inertia will continue move in that direction. This will cause the pressure on the right to continue to increase and the pressure on the left to continue to decrease, beyond the equilibrium value.

    As this continues the chunk of air exeperiences a net force to the left, which works to decelerate the chunk of air. As the difference in pressure to the left and right increases, the deceleration increases, until finally the chunk of air is brought to a stop.

    At this point the chunk of air is stationary with the pressure on the right being greater than the pressure on the left. The situation then repeats in reverse, each oscillation going through this cycle.
  16. Aug 6, 2011 #15
    Yes,this seems clear..I'm starting to draw a clear picture right now.and a tuning fork is a good example of this (it affects the pressure) right?
    Thanks very much for this clear explanation and now I can ask some more clear and definite questions:
    1-Why do sound travel through solids faster than liquids faster than gases??I think it has somethin to do with the intermolecular forces.

    2-why do longtudinal waves travel in gases only?I think it has somethin to do with the intermolecular forces too.
    3-the waves of the surface of a cup of water are transeverse while in depth,there exist longitudinal; waves..so why?

    4-on tossing a stone in a pond..why do we consider the postition of the stone as the first crest??what is the wave length then?where are crests and troughs in this case..
    I can't deal with these kinds of problems..It would be great if you use some illustrations

    Thanks very much..
  17. Aug 7, 2011 #16


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    As Whitham (1974) pointed out, many (if not most) waves are not governed by the "wave equation"!

    (He gives a much broader definition, that a wave is "a recognizable signal that is transferred from one part of a medium to another with a recognizable velocity of propagation.")
  18. Aug 7, 2011 #17
    Try reading some here:


    and look at the illustrations and explanations under each.

    You can also look up TRANSVERSE WAVE AND LONGITUDIONAL WAVE if interested.

    Waves in general carry energy ( a disturbance) from one place to another. It turns out some waves required a medium, like water waves, and others don't, like light (an electromagnetic wave). Sometimes you can see them (by eye) sometimes not. For example you can see light (waves) from the sun but not higher frequency waves from an X-ray machine.
  19. Aug 7, 2011 #18


    Staff: Mentor

    Yes, the intermolecular forces are given by Young's modulus (stiffness), the stiffer the material the faster the speed of sound. The other thing that affects the speed of sound in a material is the density of the material, the lighter the material the faster the speed of sound.

    Longitudinal waves travel in liquids and solids also. Since a fluid deforms under shear stress it is very hard to get transverse waves in a fluid.

    I don't think this is correct. Do you have a reference?

    The definition of wavelength is unchanged, the crests are the tops of the ripples and the troughs are the bottoms of the ripples. I suspect I am not understanding what you are actually asking.
  20. Aug 14, 2011 #19
    our text book of physics :(
  21. Aug 15, 2011 #20
    Re electromagnetic waves. They have length and amplitude but do they have width or what would they look like end on? Waves on water are obviously 3d because the medium they propagate through is 3d. How can a massless wave be 3d.
  22. Aug 15, 2011 #21


    Staff: Mentor

    The wave equation for EM involves 3 dimensions of space and 1 dimension of time, so I would say it is 4D. I don't know what mass has to do with it.
  23. Aug 16, 2011 #22
    is position in those 3D's definitive? Or a probability of where it is?
  24. Aug 16, 2011 #23


    Staff: Mentor

    Depends if you are doing classical EM (Maxwell's equations) or quantum EM (QED).
  25. Aug 16, 2011 #24
    I'll go with QED, since it's newer and puts a magnifying glass on what Maxwell's equations are looking at.
  26. Aug 16, 2011 #25


    Staff: Mentor

    Then it's a probability.
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