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The Wavelength of light

  1. Jul 31, 2015 #1
    Hi
    1)Is this sentence right?
    When we say for example the wavelength of red light in vacuum is 750 nm it means that the length of electric field and magnetic field that are perpendicular in the propagation direction of wave is 750 nm in one cycle?
    2) We say that oscillating charges in antenna produces electromagnetic wave. Does the distance that electromagnetic wave travels in matter depend on the speed of oscillating charges in antenna? if yes what is the formula?
    If no how is the calculation of distance?
    Thanks
     
  2. jcsd
  3. Jul 31, 2015 #2

    blue_leaf77

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    1) Yes that's right.
    2) This question is not very clear to me, by "distance that electromagnetic wave travels in matter", do you mean the penetrating depth? The relevant quantity when talking about penetration depth is the medium absorption coefficient, which is a function of frequency of the incident wave. Since the frequency of the emitted wave must be related to the rate at which the emitter (antenna) is driven, the amount of absorbed energy in a given distance must be dependent on the frequency of the emitter. As for the formula I suggest that you look up about complex dielectric function. Note that any formula you will likely find will always be expressed as a function of the EM wave's frequency.
     
  4. Jul 31, 2015 #3
    Thanks for your answers.about the second question for example an station wants to broadcast radio waves. I want to know do they calculate how far from the emitter the wave can be received?
    and is it possible to stop electromagnetic wave completely?
     
  5. Jul 31, 2015 #4

    blue_leaf77

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    I can't help for your first question. For the second, what do you mean by stopping completely, is it like you want to protect certain volume in space from outside EM radiation? There has been the so called EM wave shielding where a metallic material is used to prevent the penetration of long wavelength EM wave, another method might include the structures used in the so-called anechoic chamber. For the latter, the wave is absorbed by large amount rather than it is reflected as in the former.
     
  6. Jul 31, 2015 #5
    This really depends on the frequency of the radio waves that are transmitted, as mentioned by blue leaf77. Go to this link for more details:
    https://en.wikipedia.org/wiki/Radio_propagation
     
  7. Jul 31, 2015 #6
    Simple answer: it's complicated.

    Once emitted, the radio wave goes on indefinitely....getting weaker with distance from the emitter. EM waves might even reach us from other galaxies. SETI is looking for such and if you have ever seen antenna arrays spread out, and I mean multiple antennas perhaps many meters in diameter, that's an extreme of technology.

    Question is: can the receiver separate the desired signal from others and then detect such a signal amid noise. How much money so you want to spend on a receiver?

    What counts for a radio station: height of the transmitter and receiver, frequency [as already posted] and power of the transmitter and the signal discrimination ability of the receiver.

    For more on radio station type receivers, you can skims these within five minutes each and get some good insights.....even some illustrations.

    http://www.dummies.com/how-to/content/radio-electronics-transmitters-and-receivers.html
    ....a few basic concepts discussed

    https://en.wikipedia.org/wiki/Receiver_(radio) [wide range of receiver types discussed]

    https://en.wikipedia.org/wiki/Crystal_radio [Some great illustrations of old receivers here.]
     
  8. Aug 1, 2015 #7

    Gaz

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    The oscillating charge determines the frequency of the wave being produced and different frequencies travel through are absorbed reflected refracted differently with different matter I agree with finny it is complicated depending what you mean by matter. Buildings hills and even trees can effect the signal and satellite TV can be effected by rain if your signal is weak. What is pretty cool is if your installing a TV aerial and surrounded by large buildings blocking the path to the transmitter you can try pointing your aerial at other large buildings to get a signal that's being reflected of them. This works i have did it myself many times. This is not scientific answer btw this is just a TV aerial installers observations =)
     
  9. Aug 2, 2015 #8
    You've been misinformed at least once, so... No. The distance from consecutive peaks or troves would be 750 nm for either field.

    The field doesn't just disappear at some point. In the "far field"--meaning greater than perhaps 4 to 10 wavelengths away, the field drops off as inverse square in strength with the distance. To be more precise, the power flux per unit area decreases with inverse square distance. The distance at which it can be useful dependes on signal encoding, noise, and the sensitivity of the receiver.
     
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