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Idea that EM radiation propagates through space in straight lines

  1. Jun 27, 2008 #1
    OK so I'm familiar with the idea that EM radiation propagates through space in straight lines at a uniform speed, that of light.

    The idea that light moves in waves, that each wavelength carries a uniform energy and that the wavelength differs yet because the same speed is maintained each burst of radiation will carry differing amounts of energy per unit time.

    I know that the size of the wavelengths is supported by other evidence like the resolving power of electron microscopes.

    What I dont understand is why light moves this way. Why a wave? Doesn't that defy the idea that it ought to be moving in a straight line? Can anyone help me to understand this?
     
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  3. Jun 27, 2008 #2

    ZapperZ

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    Re: Light

    If you have a problem with thinking of how "waves" can move in a straight line, look at plane waves in water (example would be the waves reaching the beaches in an ocean). You'll see the wavefronts parallel to each other, and they move in a single direction, i.e. a straight line.

    Not only that, if you write down the wave equation for plane waves, the wave vector tells you the same thing. So having wave property and moving in a straight line are not contradictory.

    Zz.
     
  4. Jun 27, 2008 #3

    Andy Resnick

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    Re: Light

    Arnan,

    You need to re-examine your assumptions. EM radiation does not propagate "through space in straight lines at a uniform speed, that of light". Simple counterexample- a radio antenna, the radiation pattern emerging therefrom, and the radiation pattern in the far-field.

    In addition, the sentence "I know that the size of the wavelengths is supported by other evidence like the resolving power of electron microscopes." makes me wonder where you are getting your information.
     
  5. Jun 28, 2008 #4
    Re: Light

    In answer the resolving power of the electron microscope is limited by the wavelength of an electron, however many nanometers, that in itself seems a bit odd to me considering that the electron is a sub atomic particle as far as I know and not EM radiation. Why atomic breakdown resulting in the shedding of a particle should be characterized as a wave is unknown to me.

    The idea of a wavefront is not particularly helpful to me either because it implies that the wavefront has an area. When radiation is emitted it may be shed in all directions but I did not imagine that to be spherical pulses centered on the radioactive object. Rather I imagined a huge number of rays being shed in all directions, point to point.

    When atoms emit light in specific quanta commensurate with the change in energy of its electronic particles is this a ray with a vector, albeit a ray comprised of a wave, or is it something else.

    My problem is one of visualization. If you were able to zoom in on a ray of light would you see light weaving back and forth while maintaining an average straight line or not?
     
  6. Jun 28, 2008 #5

    ZapperZ

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    Re: Light

    You are trying to mix classical E&M with quantum mechanics. If you don't know either one very well, this is a lethal combination that will get you going around in circles, which is what you are doing now.

    The "wavefunction" for free electrons can be easily derived if you know how to write down the Schrodinger equation for it. In fact, this is the first thing we teach students in an intro QM class. It has nothing to do with "atomic breakdown", because it doesn't care where the electrons come from - and for your information, the electrons in a scanning electron microscope typically come from thermionic emission process from something like a tungsten.

    When such free electrons are moving, quantum mechanically, the description of such a system are described via plane wave states. The problem now is that you are mixing your idea of classical waves with the quantum mechanical description. Plane waves aren't defined by "area". That has never been a requirement - look at the mathematics describing it, if you don't believe me. What it DOES define is a direction "k", which is the wave vector. The result often indicates a "dispersionless" wave. A laser is a typical example of a plane-wave source (it is also a coherent source). Do you see the light going in all directions?

    Zz.
     
  7. Jun 28, 2008 #6
    Re: Light

    OH DEAR! i truly respect what a fine thing you have asked,but according to my little knowledge of physics, i would like to give an example for that
    Yes u r absoluetely correct that light moves as a wave. now suppose a single wave length as an entity. And the light now you can visualise as the motion of that entity in single direction. Have you ever learned or watched that you release light from a light source and it moves in a curved path?
     
  8. Jun 28, 2008 #7
    Re: Light

    You are telling me that the wave idea is the result of a mathematical formula that may be expressed as a wave rather than it being a literal wave shaped ribbon of energy moving through space?

    Note that I dont have a problem with your explanations, its just that it sounds like im going to have to abandon my physical, visual representations to understand things more accurately. It will be interesting to try to understand physical phenomena in a purely mathematical way but it will lose its meaning if I cant translate it into something more intuitive even if I understand the maths.
     
    Last edited: Jun 28, 2008
  9. Jun 29, 2008 #8

    ZapperZ

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    Re: Light

    You will realize that, as you learn more physics, that your intuition can be severely wrong and severely limited.

    Zz.
     
  10. Jun 29, 2008 #9

    jtbell

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    Re: Light

    Yes.

    Well-chosen pictorial representations can be very helpful in understanding something, but in electrodynamics and quantum physics (for example), you usually have to think of them as metaphorical in some way, rather than as literal truth.
     
  11. Jun 30, 2008 #10

    Andy Resnick

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    Re: Light

    I think a problem here is thet you are asking very deep questions, the answers to which may be more complex than you expect.

    First- the concept of wave-particle duality. This goes towards your questions regarding electron microscopy and the idea of expressing light as a wave, when it can apparently propogate in straight lines.

    The answer here is that real objects can be described either in terms of point particles or as continuous fields. The two descriptions are mathematically interchangable (analogous to Fourier Transforms), in spite of the "common sense" paradox involved. Whether or not an electron *is* a particle, or light *is* a wave is not the point- there are perfectly good physical descriptions for both in terms of particles or in terms of waves. Which one is used is, to some degree, arbitrary.

    To be sure, some physical phenomena are more simply expressed by using the wave formalism (polarization, interference, diffraction, aberrations, etc) while others are more simply expressed in terms of the particle formalism (scattering, absorption, refraction, etc). Most people encounter cognitive dissonance when they try to explain for example, diffraction in terms of particles. It can be done, but one must be careful. The same holds true for light scattering- all that regularly-appearing nonsense about light being absorbed and emitted by individual atoms as it propogates merrily through a medium.
     
  12. Jun 30, 2008 #11
    Re: Light

    Don't worry, only good things come from studying physics as it is truly formulated.

    The translation problem will be resolved for you, and you will get to keep (actualize) your intuition.

    As for your original question:

    The first part of the answer is that EM waves are transverse, which means that the oscillations occur perpendicular to the direction of travel (like a wave on a string). Here is an illustration:

    http://www.wave-guide.org/images/em-wave.jpg

    Notice that the E field and the B field are perpendicular to each other and to the direction of propagation. The reason this happens is because a changing magnetic field creates an electric field, and a changing electric field creates a magnetic field, and this process continues wavelength after wavelength.

    Michael Faraday invented the modern concept of electric and magnetic fields without mathematics. He understood the way that the fields were oriented around current carrying wires, and even predicted that light was an EM wave (but had no basis for doing the calculations to prove it).

    James Clerk Maxwell created the correct mathematical formulation of the E&M fields that is still used today. As an aid for accomplishing this he also had an enormous pseudo-mechanical intuitive model for the fields, which is no longer in use. This model “consisting of tension along the lines of force and pressure in all directions at right angles to the lines of force”, etc.

    The bottom line is that doing the mathematics is often easier and more intuitive than a long prose explanation would be.
     
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