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Electromagnetic wave?

  1. Aug 20, 2008 #1
    Electromagnetic wave???

    Wich experiments show that an electromagnetic wave is a wave?
    And what kind of wave? How i think of it? Sure not like a wave on the ocean, there is no ocean there. So?
  2. jcsd
  3. Aug 20, 2008 #2


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    Re: Electromagnetic wave???

    I'm not understanding your question here. Surely an electromagnetic wave, by definition, is a wave. This is true by definition without proof!

    Do you mean instead:
    How do we know that electromagnetic waves exist?
  4. Aug 21, 2008 #3
    Re: Electromagnetic wave???

    Yes, that´s right defender, how do we know?
  5. Aug 21, 2008 #4


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    Re: Electromagnetic wave???

    Because our cellphones work. :rolleyes:

    And our wi-fi networks, and our microwave ovens, and our radios, and our TVs (at least the ones that don't use cable)...
  6. Aug 21, 2008 #5
    Re: Electromagnetic wave???

    We know that an electromagnetic wave is indeed an 'wave' since it shows diffraction patterns. Slits or crystals(depending on the wavelength) can serve as diffraction gratings.

    As for your second question, I would say that an electromagnetic wave is a spatial & temporal oscillation of 'the electric and the magnetic field', just as what we usually call 'wave' is a spatial & temporal oscillation of 'the local displacement of the underlying medium'.
    The essence of an 'wave' lies in the 'spatial & temporal oscillation' originated from the equation of motion(the wave equation).
  7. Aug 21, 2008 #6
    Re: Electromagnetic wave???

    I recommend you read "Schrödinger's Kittens" by John Gribbin. On page 1 he discusses the most famous experiment that demonstrates the wave properties of light - Young's double slit experiment. Imagine two narrow slits in a board, close together. Shine a light on them and you get a pattern of dark and light lines on the screen beyond that can only be explained by assuming light is a wave. The waves coming from both slits interfere (wave peaks sometime coincide, sometimes clash with troughs, leaving the light-dark-light-dark pattern).


    Faraday thought the universe was filled with a material medium (like the ocean!) that carried the lines of force in the electromagnetic field (he invented those concepts as well). He called this "ocean" the plenum. Think of an olympic* rower; he pushes on the water sending a force through the water and rocketing backwards himself. In that way, Faraday thought of force progressing through the plenum. He thought of the plenum as consisting of microscopic objects, like water molecules, which were the fundamental mechanism of force progression (by action/reaction -- Newton' s third law). He considered light to be due to regular (wave-like) motions in the line-ordered force-carrying particles of the plenum. Phew! Take a pretty picture break:


    Now it gets complicated. There is no plenum. Therefore you cannot visualise directly the reality of what is going on. You can imagine water waves easily because you have experienced the material medium of water and the movements in it.

    Notice the above responses to your questions mostly point to the visual effects of EM waves (TV picture, diffraction patterns on the screen...) Visualising these is no problem (you can see them!) But how can you visualise the waves directly? How can they be there when there is nothing material to support the waves, you ask? Gribbin suggest the only way is to visualise what is going on with models and analogies (realising that they are, and can only be, models and analogies) and trust in Maxwell's equations to give you the best account of EM waves because they predict the results of experiments (e.g., the exact position of wave peaks in diffraction patterns). Gribbin (p.66) makes this clear.

    He suggests you visualise an EM wave through waves in a stretched rope. Remember that a changing electric field generates a magnetic field, and vice versa. Shake the rope so you get vertical ripples, think of that as the electric field. Because it changes it generates a magnetic field, think of that as at right angle to the changing electric field. That, in turn, generates the electric field. The two changing fields march hand in hand, down the "rope". To improve (?) Gribbin's analogy, take away the rope, but keep the ripples. Voila! Electromagnetic waves propagating through the non-material medium of empty space. David Blaine eat your heart out.

    Note, as well as trying to visualise waves in a non-existing plenum you also need to visualise fields of force in empty space:


    Not an easy task, visualising this stuff. It occupied the best minds in physics between, and including, Faraday and Einstein, before an acceptable picture was cobbled together. Conveying that picture is not easy, and physics teachers need to try harder in conveying it to the uninitiated (as the drop out rate in physics shows!)

    P.S. The double-slit experiment also provides the neatest entry into the even greater mysteries of the quantum world. But that's another story -- with good tellings by Gribbin for the layman, and Feynman in his lectures.

    * Go Britain!
  8. Aug 21, 2008 #7


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    Re: Electromagnetic wave???

    The first really clear indication that electro-magnatism is a wave was the fact the Maxwell's equation for the electric and magnetic fields imply the wave equation.

    Though the old SPSS highscnool physics program had a cute experiment on this: A water wave, moving at an angle into an area of lower depth (so slower wave speed) changes direction closer to the normal, obeying Snell's law. A ball (particle) will do that if moves down a slope (so faster speed). Since we know that light moves slower in glass than in vacuum and obeys Snell's law it follows that light has wave properties.

    Of course to say something is a "wave" does not necessarily mean it is a wave in any material medium. Light is a wave in the electro-magnetic field.
    Last edited: Aug 21, 2008
  9. Aug 21, 2008 #8
    Re: Electromagnetic wave???

    Why was Young's experiment, performed in 1801, not the first "clear indication"?

    If the EM field is not a material medium how would you describe it? Here's something i dreamt up, not sure if it holds water (or plenum:-):

    Can the EM wave be thought of as a wave of *potential* energy? Can one use a ball on a step as a metaphor? The ball on the step has no obvious energy (it just sits there), but it has, it has potental energy because it converts to obvious (kinetic) energy on falling off the step. With the EM wave it does nothing until it "hits" the radio mast, so its all potential energy up until that point. Just as the potential energy of the ball obviously needs no material foundation, neither does the EM wave or field.
  10. Aug 21, 2008 #9


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    Re: Electromagnetic wave???

    Young's experiment indicated that light had to be a wave phenomenon, but it didn't indicate anything about the nature of the wave. At that time the concepts of electric and magnetic fields hadn't even been developed (except perhaps in a very vague sense), let alone been connected to light.

    It wasn't until the 1860s that Maxwell derived the wave equation for electric and magnetic fields, calculated the speed of electromagnetic waves from measurements of electric and magnetic constants, and noticed that that speed equals the measured speed of light within experimental accuracy.
  11. Aug 21, 2008 #10
    Re: Electromagnetic wave???

    The first real experimental evidence that showed the existence of Maxwell's electromagnetic waves was provided by Heinrich Hertz in 1887.

    He set up a coil-driven spark generator to radiate radio waves (which at the time was just considered invisible radiation), and used a circular antenna to receive them. Using Maxwell's theory, he deduced that this radiation consisted of electric and magnetic fields. He also demonstrated that this radiation could be reflected, polarised and produce interference, which are inherent properties of waves. Futhermore, he measured the speed of the radio wave to be similar to what Maxwell had predicted for light.

    Wikipedia quote:

    "It's of no use whatsoever[...] this is just an experiment that proves Maestro Maxwell was right - we just have these mysterious electromagnetic waves that we cannot see with the naked eye. But they are there."
  12. Aug 22, 2008 #11
    Re: Electromagnetic wave???

    Good points. But Moris asked "Which experiments show that an electromagnetic wave is a wave?" He didn't say "*all* electromagnetic waves or *except light*. So I think 'Young's experiment' was a valid answer, without more input from Moris.

    But that's a theory. Moris was asking for an *experiment*. The following is quite useful on the history:


    "In 1887, Hertz finally achieved the sought after experimental confirmation."

    Found this after a quick Google search:


    It would a good school project to repeat this experiment.
  13. Aug 22, 2008 #12
    Re: Electromagnetic wave???

    Yeah, Young's double slit is the answer. Interference and diffraction.

    You can also take Maxwell's equations and show that E and B must satisfy the linear wave equation, implying they are waves.
  14. Aug 22, 2008 #13
    Re: Electromagnetic wave???

    What is most confusing here, is why electromagnetic waves are transverse, which I cannot understand. Is it really that they are transverse in space, that is, when a light ray beams at my eye, it travels through the air as a wave with a physical extension in the z-direction (if the light ray travels horizontally to me, and z is in the up-direction)?

    Also, I cannot sea how any of the experiments here explain why the wave is transverse. The experiment with interference is for me that they could be as well be only longitudinal. At least, it is not explained in a way that is easy to understand... Is there an experiment that proves the transverse nature of the EM waves, with sort of high pedagogical power?
    Last edited: Aug 22, 2008
  15. Aug 22, 2008 #14


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    Re: Electromagnetic wave???

    The existence of polarized light proves that EM waves are transverse. Polarization is impossible with a longitudinal wave.
  16. Aug 23, 2008 #15
    Re: Electromagnetic wave???

    Thomas Young explained polarisation as well! Got me to thinking if there was a biography about him. Turns out there is, with the longest & most descriptive title I've ever seen!

    "The Last Man Who Knew Everything: Thomas Young, the Anonymous Polymath Who Proved Newton Wrong, Explained How We See, Cured the Sick, and Deciphered the Rosetta Stone, Among Other Feats of Genius" by Andrew Robinson

    Simon Singh provides a great review of this book (I accessed it from http://en.wikipedia.org/wiki/Thomas_Young_(scientist)), recounting how Young 'might have been' inspired to invent the ripple tank to demonstrate interference in water waves:

    "When I was a student at Emmanuel College, Cambridge, where Young also studied, I was told that this demonstration was inspired by the college's famous duck pond. The story goes that Young saw two ducks swimming side by side, leaving behind a complex pattern of interference from the two sets of water waves."
  17. Aug 23, 2008 #16
    Re: Electromagnetic wave???

    Okey, I'm satisfied with that when it comes to the transverse nature. I'll go and study polarization then.

    But still - what kind of transverse are we talking about here? Are the waves really a wave in space, with, as I asked above, a physical extension in z-direction? So that you can say that, if the wave has an amplitude of 1 nm, then the wave actually goes up 1 nm on the hills of the wave, and down 1 nm in the vallyes of the wave?

    Interesting about Young however! And the wavemotion link was very useful! :smile:
  18. Aug 23, 2008 #17
    Last edited: Aug 23, 2008
  19. Aug 23, 2008 #18


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    Re: Electromagnetic wave???

    No, what is oscillating are the magnitudes of the electric and magnetic fields at a given point (measured in volts/meter and tesla respectively), not displacements in position. Maybe I can find the diagram I attached to a posting here a couple of years ago... ah, here it is, in this post.
    Last edited: Aug 23, 2008
  20. Aug 23, 2008 #19
    Last edited: Aug 23, 2008
  21. Aug 23, 2008 #20
    Re: Electromagnetic wave???

    Waoh! This is a revolution for me. :smile: Why have all my physics teachers deceived me on this point? :mad:

    Follow-up: Since the E-field describes the force per unit charge in each point, you could say this is just the property that the z-axis describes, just as pressure in air is the property which the z-axis describes when plotting the propagation of sound (but sound is a longitudinal wave). So what's really the difference then in form between a sound wave and an EM wave?
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