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Wave theory of light

  1. Jan 28, 2004 #1
    Hi, I'm new to the forums. I know next to nothing about anything related to physics.

    My question is about light.

    Light as a stream of particles (photons) kind of makes sense to my unscientific brain.

    But light as waves?

    Ocean waves are surface waves propogated across the surface of the water. Sound waves are pressure waves propogated through the air, or through whichever medium happens to be carrying the sound waves.

    How can light be waves? What is the medium of propogation? How can you have waves in the empty vacuum of space? What is it that's waving?
  2. jcsd
  3. Jan 29, 2004 #2
    Light is believed to be wave beacause it behaves like a wave in familiar(to us) scales. Light is an electromagnetic wave, changing electric field produces a magnetic field and changing magnetic field produces an electric field.
    Your question was in the minds of scientists from Newton to Maxwell. It was believed to propagate in a medium called "ether".
    Then Michelson-Morley experiment came. Light had the same speed every direction. So there couldn't be any kind of ether.
    Today, in Quantum Theory, quanta of light is photon. Photon is surely a particle (according to light amplifier experiments I think). But you know, it isn't a "particle" like a soccer ball. It is a particle that also behaves like wave. I hope this was helpful.
  4. Jan 29, 2004 #3
    Light is its own medium. A water wave needs a medium to propagate for the same reason that a friendship needs two people between which to exist. A water wave is a way of describing something that water does.

    But we know that light is real without having something to exist within. It's not like friendship, it's like sweaters. Take away the people, you've still got sweaters. Light can propagate through air, water, plastic...or nothing at all. Here's how:

    Maxwell's Equations say that magnetic fields get curly whenever there's an electric current present...or whenever there's a changing electric field. They also say that electric fields get curly whenever there's a changing magnetic field. So in the wave picture of light, here's what happens: you've got a source made of charged particles, swirling around and making electromagnetic fields. Now an electric field by itself will die off pretty fast, and so will a magnetic field by itself. But if you've got both of them together, they can work together, sort of piggy-backing off of each other's curliness. Curly E-field creates a change in the B-field, which makes it curly, to make a change in the E-field, which makes it curly... electromagnetic waves are one possible behavior for E and B-fields allowed by Maxwell's Equations, which are based on sound observations of electromagnetic phenomena. It is one of the triumphs of E&M that the predicted speed of these waves, equal to the inverse of the square root of the product of two fundamental electromagnetic constants, is exactly equal to the speed of light in a vacuum.

  5. Jan 30, 2004 #4


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    That's very good. I like it.
  6. Jan 31, 2004 #5
    What a creativity!
  7. Jan 31, 2004 #6


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    wave and particle?

    'Light', as in electro-magentic radiation, behaves like a wave in certain experiments that you can do; for example, constructive and destructive interference (the colours on a soap film, rainbow colours from a CD in sunlight, etc).

    It also behaves like a particle in other experiments, which you can also do; for example, the photoelectric effect.

    How can it be both?

    Here's the really fascinating thing: there's an experiment you can do which shows light behaving as both a wave and a particle! It's called the 'double-slit experiment', and is one of the easiest experiments to do, yet one of the most curious.

    Once you've got your mind around that, consider this: you too have a wave nature! (So do I, Halls, kish, rocket, etc; even the Earth). Our wavelength is very small however.
  8. Feb 1, 2004 #7
    Re: wave and particle?

    Wave-Particle duality is not a problem now, thanks to QED. It explains all of the wave-like behaviours of photons.
    They should have thought that Louis de Broglie was mad, but yes, that's real.
    More info about nature of light: http://www.encyclopedia.com/html/section/light_thenatureoflight.asp [Broken]
    Last edited by a moderator: May 1, 2017
  9. Feb 1, 2004 #8


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    "QED" as in "Quantum Electrodynamics".

    This is something which many members of PF have asked about, and are curious about. Do you have a good, non-technical reference? One which explains QED, how it handles "wave-particle duality", and explains the wave-like behaviour of photons? The history is also fascinating; how special relativity was added to quantum mechanics, and how experiments of extraordinary accuracy have confirmed QED, to, what, 9 decimal places?

    I for one would sure appreciate having such a reference to hand!
  10. Feb 2, 2004 #9
    The best reference is QED by Richard Feynman himself. To think and wonder and to be shocked by the nature.
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