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Another light thread more electromagnetic

  1. Dec 21, 2006 #1


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    I just hought about it, and why is there not a correct definition of light?
    Some says they are particles, others say they are waves. Some say they are energy "packages"...

    If they are waves, they need something to travel through, as i understood as the elecromagnetic field that is everywhere. Does this field exist in places with no mass at all, or whats making this field?

    If they are particles, and since they go in all directions there must be infinite particles shooting out, which I consider impossible.

    And I do not understand this photon "packages" that my science book says. What do they mean about that, and what are these packages?
  2. jcsd
  3. Dec 21, 2006 #2


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  4. Dec 21, 2006 #3
  5. Dec 21, 2006 #4


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    sometimes light exhibits wave-like properties (interference, etc.) and in other contexts light exhibits particle-like properties (photo-electric effect). we don't know precisely what light is, but we know some (many) of the properties of light. some of these properties seem, at first blush, contradictory to each other (is it a wave or is it a stream of particles?), but nonetheless, that is what we observe and this physics of the wave-particle duality was built around this fact.

    no. they don't. and, in fact, our best measurements (Michaelson-Morley experiment and descendents) cannot find any evidence that they need anything more than utterly empty space to travel through.

    the field is the same field that says opposite charges attract and like charges repel and that this force varies as the inverse (or reciprocal) of the square of the distance between the charges.

    imagine that you and i are standing some distance apart and facing each other. you're holding a positive charge and i am holding a negative charge and that we both are restricting our charges so they cannot move toward each other but they can move up and down and left and right (just not forward or backward). so i move my charge up a meter. since your charge is attracted to mine, your charge also wants to move up a meter and you allow that. then i move it down and your charge follows it down. now i move it to my right (your left) and your charge moves toward your left. then to my left (your right) and your charge follows it.

    now i move my charge up and down repeatedly and your charge follows it up and down. that is an electromagnetic wave that originated with me moving my charge around and that wave moved toward you (at the speed of propagation of E&M waves which is "c") and caused your charge to move. in a very real sense, my moving charge is a "transmitting antenna" and your moving charge is a "receiving antenna". if, somehow, i could move my charge up and down a million times per second, you could tune your AM radio to 1000 kHz and hear a signal (a silent carrier). if i could move it up and down 100 million times per second, you could tune it in with your FM radio just between the 99.9 and 100.1 settings (provided no other stations were close by). if i could move it up and down 500 trillion times per second, you would see it as a blur of orange colored light.

    that is what light is (from a wave-property perspective) and it required no medium for these waves to travel. they just are there because unlike charges attract and like charges repel - there need be no medium in between for that to happen.

    there are other phenomena that light exhibits that make it appear like it's made up of particles (such as the photo-electric effect). seems like a contradiction, but both behaviors are exhibited by light and all the physics is, is an attempt to coherently describe and explain both demonstrated properties.

    no, in the photo-electric effect, they can actually count the particles (or at least compute the number). given a radiant intensity (watts/meter2) and an average frequency of the light, we can determine how many particles per second cross the square meter that is perpedicular to the direction of radiation.

    this photon "package" is a semantic that your science book is using to describe the particle-like qualities of light (or any other electromagnetic radiation).
    Last edited: Dec 21, 2006
  6. Dec 21, 2006 #5


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    Thanks for reply! greatly appriciated, I did not understand the wave explanation though, many of my questions remain unanswered, but i'll look at the links.
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