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Manmade Sun

  1. Feb 14, 2009 #1
    This is purely a question I know little or nothing about light and the whole EM spectrum for that matter I was just wondering something. Why are visible light waves so puny compared to other frequencies on the EM spectrum. What I mean is that I can send a pulse to an antenna and emit lets say microwaves which travel so far they can be picked up on the moon. Now if I can do that with microwaves why can't I do it with light?

    When I turn on the light bulb do light waves actually travel thousands of miles through the air only they are so sparse that they cannot be picked up by the human eye? With that question in mind heres another thing I was wondering. Since information can be encoded (modulated) into radio and microwaves can other EM waves be similarly modulated? For example could I broadcast messages over EM waves in the visible light spectrum? I've never heard about gamma rays being used for this either.

    Sorry for this flurry of questions but heres one last thing I was wondering about waves. Since light waves can be beamed in a straight line as in a laser can other EM waves be beamed like a laser? Could I say beam radio waves to my next door neighbour in a straight line without the possibility of anyone intercepting those radio waves without getting right in front of the radio beam? Could this also be done with longitudinal waves like say sound?
     
  2. jcsd
  3. Feb 14, 2009 #2

    mgb_phys

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    I suggest you don't play with lasers

    How far away are the stars that you can see with your bare eye?

    This message is almost certainly being sent over light = fibre optics.

    gamma rays are strongly absorbed by air and most materials, they are also difficultto generate and focus.


    Yes that's what's done with microwaves, they do spread out more than light but that's just because it's not worth the cost of making a large enough dish to make a more collimated beam
     
  4. Feb 14, 2009 #3
    Thanks for the concise answers. I didn't even think about fiber optic cables. What I had in mind was messages conveyed on laser beams through the air but if it can be done through fiber optic cables then I assume it can be done through the air.

    I assume its hard a difficult task intercepting these direct microwave transmissions then.
     
  5. Feb 14, 2009 #4

    mgb_phys

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    You can do laser beams through air - it's called 'free space optical' but compared to microwaves it's a lot more susceptible to atmospheric conditions (rain fog etc) and slower and a lot less reliable than fibre

    The spread out of a beam depends on the size of the lens/dish, since a lens is so much larger than the size of a visible wavelength it's easy to make a very colimated beam. A microwave antennae 1-2m diamter doesn't give a very collimate beam - it might be 10s of m wide at the next tower, you try and collimate as much as possible because it reduces the power needed (more power/m^2 at the recevier) rather than snooping.
     
  6. Feb 14, 2009 #5
    Before the invention of the LASER there was the MASER - microwave amplification by stimulated emission of radiation. In other words, a "microwave" laser.
     
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