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Density And Radio

  1. Nov 17, 2006 #1
    For long wavelengths like radio is dense material better to block, or shield, as it is for short wavelengths like x-rays?

    You can block out the photons of the visible part of them electro-magnetic spectrum by pulling the curtains closed, so how come radio waves like your AM radio, cell phone, and television can work through brick, and ignore the blockages? Is this to do with the wave-length of radio, as they are all made of photon waves you would think they would be blocked too. See here for what I've been reading : http://imagine.gsfc.nasa.gov/docs/science/know_l1/emspectrum.html
  2. jcsd
  3. Nov 19, 2006 #2

    Claude Bile

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    It is erroneous to assume that since all EM radiation is made from photons it ought to behave the same. Different parts of the EM spectrum will interact quite differently with matter.

    In the case of radio waves, the energy of the wave is typically not enough to induce any resonant effects with atoms, so radio waves can pass through most things. Radio waves can also diffract around most objects due to the long wavelength. Conductive surfaces or Faraday cages are typically used to shield radio waves.

    Cell phone radiation cannot pass through brick, it enters your home via diffraction. Television actually works by detecting a signal on your roof via an antenna and converting it to an electrical signal, unless you have cable in which case the electrical (more correctly, optoelectronic) signal is transmitted directly to your home.

  4. Nov 29, 2006 #3

    Can you explain how this refraction works, this phen. that allows radio waves to seemingly pass through things like brick. A website perhaps, with some images?
  5. Nov 29, 2006 #4
  6. Nov 30, 2006 #5

    Yes he did say "diffraction". Now back to microwaves like mobile phones and houses. Why does cell phone rad. not penertrate brick, and what part of the diffraction principle enables them to do so? The bending around objects, or the spreading of waves through a gap or apature. (Thanks cesiumfrog for the page).
  7. Nov 30, 2006 #6

    Claude Bile

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    The rule of thumb is that if an aperture (or obstacle - it works both ways) is roughly the same size or smaller than the wavelength, the EM radiation will diffract through the aperture (or around the obstacle as the case may be).

    This is encapsulated in the elementary 'picture' of diffraction (i.e. a plane wave incident on an aperture) when you get the wave diffracting over the full 180 degrees when the size of the aperture is nearly equal to, or less than the wavelength.

  8. Dec 2, 2006 #7
    When the wave goes through a gap, does the whole of the wave get through, so no part of the mobile phone conversation is lost?

    When you think about it it is strange, a hole less then the wavelength, and it gets though intact? Is that what you're saying?
  9. Dec 2, 2006 #8
    Can you explain how in that site above it shows waves as straight lines AND circles radiating from a point somewhere? Also how come when you look up a wave someplace else it has amplitude, I'm finding it difficult to build up a picture of waves, especially radio waves, as obviously they can't be seen.
  10. Dec 3, 2006 #9

    Claude Bile

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    Radio and Mobile phone signals are modulated, which means the information exists as a variation in frequency, phase or amplitude of a carrier wave. You therefore only need to be able to pick up the changes in the carrier wave, which means you only need to measure the wave at a single point (via an antenna).

    You lose information if the signal strength becomes attenuated to the point where ambient noise blots out the information. This is a possibility if the wave is diffracting around/through many objects and apertures.

  11. Dec 3, 2006 #10

    Claude Bile

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    This is a visual reconstruction of Huygen's principle which can be used to calculate the shape of a wavefront buy regarding all points on an existing wavefront as 'secondary' point sources of waves.

    Keep in mind that the amplitude of an electromagnetic wave is simply the maximum strength of the electric field within the wave. While we often use a physical displacement to represent the wave graphically (particularly since Em waves are transverse), this method of representation does have its limitations. A contour plot is typically more physically representative (though still not perfect) of what an EM wave looks like. Wavefront diagrams (which are essentially rudimentry phase plots) such as those shown in the website are also commonly used where the amplitude of the wave is not of interest.

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