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How does light reflect?

  1. Mar 22, 2013 #1

    adjacent

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    My concern is how does light reflect from matter.Does it simple touches it(matter) and go away?Im in middle school,so i had only studied about that.Can someone explain me what happens at the subatomic level?
     
  2. jcsd
  3. Mar 22, 2013 #2

    BruceW

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    hey, welcome to physicsforums :) Using classical electromagnetism, light is waves, and these waves depend on the type of material they are moving through. The air is almost like a vacuum, while a solid object has very different properties. So when a light wave travels through the air and encounters a solid object, it is meeting a sharp boundary. And because of this boundary, some of the wave is reflected, while some is absorbed.

    In quantum mechanics, it gets a little bit more complicated because the energy can only be transferred in packets, or 'quanta' of certain sizes. Which is where the idea of photons is useful.
     
  4. Mar 22, 2013 #3

    jedishrfu

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    Atomically, a photon strikes an atom and the outermost electron will absorb the energy and move to a higher state but it requires a photon of a specific energy to do so. If the photon energy is too low the atom will reflect it (like a ball bouncing off a wall) back.

    Photons that are absorbed aren't reflected back. When something looks green to you it means that the green light photons wee reflected back and the other color photons in sunlight were absorbed.

    An earlier thread to look at:

    https://www.physicsforums.com/showthread.php?t=211097
     
  5. Mar 22, 2013 #4

    Cthugha

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    Actually, the important post in that thread is this one:
    https://www.physicsforums.com/showpost.php?p=1585144&postcount=4

    It is important to emphasize that solids as a whole behave very differently from single atoms or molecules. Solids, especially metals show reflection, but there is no reflection at at the subatomic level for single atoms or the like.

    Reflection off metals (like in silver mirrors) usually work in terms of plasmons. Let me try to get this to a middle school level (some accuracy will be lost on the way of course): In metals, you have a lot of electrons which can move around quite freely. Electrons show some very interesting interactions with electromagnetic fields:

    1) When some electromagnetic field is applied to them, they move.
    2) When they accelerate, they also radiate an electromagnetic field.

    Now light is a very rapidly oscillating electromagnetic field. As it hits a metal, the electrons will start to oscillate, too. So they will go back and forth. However, that also means, they are permanently accelerated and stopped again. That means that they will also radiate an electromagnetic field. The superposition of all of these fields radiated back again is the reflected light beam.

    Note that there is a so-called plasma frequency. Depending on the material, there is a highest oscillation frequency the electrons can follow. If you want to "shake" them at higher frequency, they will not be able to follow. As the frequency of a light wave is correlated to its color, this gives a first hint at why some metals seem to shine in different colors.
     
  6. Mar 22, 2013 #5

    Drakkith

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    Wow, thanks for that description Cthugha. I didn't know how reflection worked until now!
     
  7. Mar 23, 2013 #6

    adjacent

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    Why does it reflects at a specific angel?
     
  8. Mar 24, 2013 #7

    BruceW

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    Again, I'm going to go with the simplest explanation, since that's the one you will most likely learn first. Light travels by a principle of least time. Meaning that if you consider a fixed start and end point, a ray of light going between the two will take the path which gets it there in the shortest time possible.

    Now this is very simple in a vacuum, because the speed of light is the always the same in a vacuum, therefore light travels in straight lines in a vacuum. But if there is some kind of material, then this will change the value of the speed of light, so the path of least time will not necessarily be the shortest path.

    Aaanyway, this "principle of least time" gives us the law of reflection, which means that the angle of incidence = angle of reflection. This makes sense intuitively, because if we assume the principle of least time, and assume the air has constant density, then the path of least distance is the same as the path of least time (since its velocity is the same over the entire journey). So what is the path of least distance? It is the path where the angle of incidence = angle of reflection (by basic geometry).

    So, what I was trying to explain is that principle of least time gives us the law of reflection. And in fact, this 'extremal principle' is a deep concept that is used in a lot of quantum mechanics.

    edit: well, I don't really know what I mean by 'deep'... I just mean that it is important in physics courses that are taught later on.
     
  9. Mar 28, 2013 #8

    adjacent

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    thanks Bruce W
     
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