1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Hey guise, Question about how antennas detect electromagnetic waves??

  1. Nov 16, 2012 #1
    Yer.. I'm lazy and tired, did not enough research and found out that the radio/electromagnetic wave makes all the electrons move back and forth when the wave oscillates between the magnetic and electric fields.... so you can see how much the current oscillates etc.

    But... um cameras work differently, the wave collapses so you get one signal of a wave and colour, much simpler.... what about quantum mechanics, don't the large wavelengths collapse as much?

    Well in general I dont know well enough how long wavelength EM-waves are detected, I've seen those pics from deep space where they detected intergalatic radiation.

    Anybody care to explain? or give links, names of phenemonas etc? I'm curios I NEED to know this.
  2. jcsd
  3. Nov 16, 2012 #2
    In order for the electrons to move back and forth, they have to absorb a large number of radio photons. However, there are so many photons in the "radio wave" that the absorption of one or two photons doesn't affect the wave as much. The radio wave changes, but it isn't really a "collapse".

    The word "collapse" is accurate only for waves that contain only one particle. If the wave has a sufficiently large amplitude, there are many particles associated with that wave. So the disappearance of one photon in the wave doesn't change the wave in a catastrophic manner.

    No they don't. Not really. The film is exposed by chemical reactions. However, chemical reactions involve electric currents on an atomic and molecular level. The formation of chemical bonds involves electric currents of small spatial extent.

    One can get a small amount of distance into this by considering chemical bonds as a type of antennae. I mean, there are differences due to the difference in scale. However, there is a correspondence between the rules of photochemistry and the rules of induction.

    In silver nitride crystals, there is a threshold in photon number required before the silver is oxidized. It takes about 10 photons absorbed before a particle of silver nitride changes color. So really, the "collapse" is divided between at least 10 photons. This means that in terms of the electromagnetic field, the photochemical process in standard film can be considered "classical". Quantum mechanics is somewhat involved in modelling the energy levels of the electrons in the silver nitride particles.

    As a heuristic condition, assume that the threshold energy for detection is about the same in antennae and film. The smaller the frequency, the less energy each photon has. Therefore, there smaller the frequency the more photons there are associated with that small amount of threshold energy.

    Radio waves have a smaller energy than light waves. Therefore, radio waves have more photons per unit amount of total energy. Therefore, the "collapse" of any one photon is less noticeable in radio wave antennae than in light wave film.

    I suggest that you try going the other way. Try to look for explanations in "classical electrodynamics" for understanding "optics". Try thinking in classical terms first, and then modifying your thinking to take quantum mechanics into account.

    For instance, start thinking of chemical bonds as being like radio wave antennae. Then, add a few changes to your picture to include the effect of photons.
  4. Nov 16, 2012 #3


    User Avatar
    Staff Emeritus
    Science Advisor

    Digital cameras work by absorbing energy from EM waves, just like antennas do. The difference is that the wavelength is so small and the energy is so high that instead of getting an AC current, we can get single electrons being knocked around by single photons. Sensors in digital cameras manipulate this to make the electrons get knocked out of one "layer" of the sensor and into another one where it is captured and later measured. The more electrons, the brighter that particular pixel is. Color images are created by putting filters in front of the sensor or even each individual pixel. Typical color cameras have a Bayer Array, which are RGB filters over each pixel. The software in the camera or on your computer can take the raw data, and knowing the pattern of the Bayer Array, convert the image to color.

    Also, don't try to mix classical and quantum physics or you will confuse yourself. The EM wave never collapses, as it is not a QM wavefunction.
  5. Nov 18, 2012 #4
    Im still not sure about, how the radio wave creates the current. Im guessing it flies trough the antenna, a certain number of photons in the wave collapse on metals making electrons jump creating current? Or can it create the current without collapsing, and how?

    Im currently reading quantum electrodynamics by feynman, and am hoping for some answer... classical physics tend to bore me a bit, at least in school... they treat us like memorizing robots
  6. Nov 18, 2012 #5


    User Avatar
    Staff Emeritus
    Science Advisor

    Don't try to mix classic and quantum physics. Just think about it as an EM wave that induces a current.
  7. Nov 18, 2012 #6
    Well... all I'm saying is that atoms are quantum mechanical, collapse of photons too? I can't see how else it functions. And how they detect ultra low energy-wavelengths. That is intergalatic radiation, since the energy is too little to cause an electron to jump from an atom in a single low energy wave.

    But thanks for the input tho
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook