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B Echo of a wavelength

  1. Jan 2, 2018 #1
    In the electromagnetic wave lengths say from x-rays to the 21 cm hydrogen line, the wave has a wave length. When it hits a receiver, eye, camera sensor whatever, it is turned into an electrical signal that can be interpreted by that device. Now we get to my question, is there an echo of that wavelength? Why I can hear you ask. If there is an echo then a generic receiver could be built that covered all wave lengths at the same time.
    I know the answer "no echo" but has anyone looked?
     
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  3. Jan 2, 2018 #2

    sophiecentaur

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    I can't be sure what you mean exactly but I can tell you that a 'signal' always consists of more than one frequency. If you can identify an 'echo' then there must be some marker on the signal (perhaps just a pulse or a beep on a radio sound signal) so that you can measure the time delay for the echo.
    If you want to measure accurately the time delay of the returning echo then the added marker (modulation) must contain a wide band of frequencies, that is a very short pulse. So your question about echos doesn't really apply to just one wavelength.
    Does that help?
     
  4. Jan 2, 2018 #3

    anorlunda

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    I too am not sure what you are asking about. Is it related to a black body? A black body would have no echo.

     
  5. Jan 2, 2018 #4
    Sorry, what I'm thinking is that if a wave after arrival at the sensor and after being detected the resultant change from the native wavelength to the image, does that image have any echo of the incoming wavelength. (It's starting to confuse me!) Let me try and explain with an hypothetical example.
    A X-ray is 'seen' by a detector and then transformed in something that can be seen by us, does that image contain anything that refers to the original X-ray wavelength?
     
  6. Jan 2, 2018 #5

    stefan r

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    A photon hits an electron bound to a nucleus. The electron leaves the atom with a velocity. The velocity includes the energy of the incoming photon.

    Echos are reflections. Photons can be reflected, refracted, and diffracted.

    The classic measurement of wavelengths used refraction. When diode detectors were invented they were placed into an array. You can also use x-ray diffraction to find the wavelength of a source.
     
  7. Jan 2, 2018 #6

    kuruman

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    I would say there is some kind of echo in the following sense. Think of the incoming EM radiation as packets of energy a.k.a. photons. Say one of these photon is absorbed by an appropriate transducer that converts it into another kind of energy. Does this mean that all the photon energy is transduced? Not necessarily. Whatever is left over heats up the absorber and is probably re-emitted as IR radiation. Isn't that an echo?
     
  8. Jan 2, 2018 #7

    stefan r

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    If a photon is adsorbed by an electron then all of the photon's energy is adsorbed. It is all or nothing. [photo electric effect]

    The electron could emit a new photon. If the electron gains enough energy it could be ejected and carry a kinetic energy. Electrons flowing through materials can heat up the material. Materials can adsorb photons and disperse all of the energy as heat.
     
  9. Jan 2, 2018 #8

    kuruman

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    A proportional counter detects photons and produces an output that is proportional to the energy of the photons. Now the energy of the photon is inversely proportional to its wavelength so yes, the output of the proportional counter contains something that refers to the original X-ray wavelength.
    Indeed all of the photon's energy is absorbed. But what happens next? Isn't that energy re-emitted as kinetic energy of the photoelectron and, if there is no photoelectron, as blackbody radiation?
     
    Last edited: Jan 2, 2018
  10. Jan 3, 2018 #9

    sophiecentaur

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    Only if the detector produces a different signal for different photon energies (as in @karuman 's post). A simple photodetector doesn't do that. It would be possible to separate photoelectrons of different kinetic energies with a magnetic field which could be easier to describe than the proportional counter but perhaps not a practical method.
    But I don't see that has anything to do with echos.(??) I guess it's just not the word you meant.
     
  11. Jan 3, 2018 #10
    Also, the detector may not absorb all the wave energy so some will reflect, some small amount. However, since the detector would be in a place that would have no blockages (otherwise it won't detect some angles and such, you need a free field to get full res). So what would happen is maybe some of that signal, whatever wavelength you observe, will reflect and go off into space or atmosphere or something, if say on Hubble or Spitzer, it would reflect and never come back so there would be no echo. Don't think you would get what you want even if you could somehow force an echo. Just my opinion of course.
     
  12. Jan 3, 2018 #11

    sophiecentaur

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    @Simon Peach
    I have a feeling that we have to take a big step backwards with this thread and try to find your present state of knowledge. Do you understand the basics of what a radio (or TV) receiver does? You can tune the receiver to a whole set of different channel frequencies and there is nothing in the final sound / picture that contains information about the frequency of the channel that is being used.
    I have to ask you why you would think this.
     
  13. Jan 3, 2018 #12
    Yes to the questions. As for the last part, I don't think it: I'm just surmising. If there was some observable "echo" then a generic sensor could be produce so that all wave lengths could be observed at the same time. Of course it would take a great deal of computing power, I image much more then we have at hand now. But it would have great impact on set up cost as only 1 lot of infrastructure would have to be set up to house it, every thing from X-ray to 21cm radio waves in one sensor.
    I think that this was the wrong forum to post this in, A more open minded and 'what if' forum would more suit this question
     
    Last edited: Jan 3, 2018
  14. Jan 3, 2018 #13

    kuruman

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    PF has a "Science Fiction and Fantasy" forum. Perhaps you should consider posting there.
     
  15. Jan 3, 2018 #14

    mfb

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    There is no detector that can detect "all" wavelengths at the same time ("all" in quotation marks because the electromagnetic spectrum is unlimited - there is no largest wavelength and no largest frequency either).

    To detect radio waves, you need something like an antenna, the incoming radio waves accelerate electrons back and forth in the antenna, this leads to a detectable signal. The efficiency of this detection depends on the wavelength of the radiation and the length of the antenna (and on its shape) - you can't even make an antenna that reacts with high sensitivity to all radio waves, and the antenna will be blind to everything with higher frequencies.

    To detect infrared radiation, cameras often have sensor arrays where they measure the temperature. That works nicely as long as the incoming radiation is mainly infrared, other wavelengths don't reach the detector or don't get absorbed by the detector. The radio waves detected by the antennas above are typically too weak to heat the sensor elements. While you could detect visible light or UV that way, you can't distinguish it from infrared by just measuring the temperature.

    To detect visible light, most sensors let the light move electrons between different energy levels. Radio waves and infrared don't have enough energy to do that.

    And so on. You need a large number of different sensors to detect the various parts of the electromagnetic spectrum.

    You also have to focus the incoming radiation in some way, otherwise you'll have no idea where radiation came from. Focusing the different parts of the electromagnetic spectrum needs completely different approaches. Something that will focus radio waves doesn't work at all with x-rays and vice versa.
     
  16. Jan 3, 2018 #15
    It must be tied specifically to some sci-fi work. We are going to close this here.
     
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