wave theory

Discussion in 'Physics' started by Phillip Lucas, Jun 13, 2017.

  1. Phillip Lucas

    Thread Starter New Member

    Mar 13, 2017
    28
    0
    I am totally new to all this fun science stuff so was wondering something about radio waves. as this question is not about designing anything I thought it would be better here then anywhere else.

    So I understand frequency is a peak to peak measurement of wave forms. I understand that most antennae are made to measure this waveform at the speed of light which radio waves travel at.
    My question is this... lets say its not a radio wave per se, that I am measuring. let say it is something that IS picked up on an antenna but is travelling slower than the speed of light. Will the receiver decrypt it or would it be bounced of as jargon?
    also if it is travelling slower than RF then the next question I have is would the antenna be able to correctly get the frequency?
    I mean if it is looking for something travelling at lightspeed and the frequency picked up is slower, would it not count the peak to peak wave and then calculate it at being a frequency based on lightspeed?

    the reason I ask this is a friend has annoyed me with a question about certain frequencies, so I want to test some projects to prove they do travel at the same speed, but with different frequencies
     
  2. BR-549

    Distinguished Member

    Sep 22, 2013
    3,163
    776
    I'm not sure I understand your question. Do you believe that the speed of a radio wave changes with the frequency?
     
  3. Phillip Lucas

    Thread Starter New Member

    Mar 13, 2017
    28
    0
    NO the friend does. He is of the belief that light is not in the rf band as Radio would travel slower than light. He stated that is why we can't use antenna to pick up light but we can use it to pick up rf....... then he went on about magnetism (got that sorted for now)... lets say it was a long chat that has made me think I want to prove him wrong....

    so the question I have is if something is traveling at light it WILL trigger the effect of a receiver, if it traveling slower then would the antenna detect it, and if it did detect it, would it detect it at the right frequency?
     
  4. nsaspook

    AAC Fanatic!

    Aug 27, 2009
    3,771
    3,917
    We can use an antenna to pickup EM energy at light frequencies. https://en.wikipedia.org/wiki/Optical_rectenna
    Changes (waves or disturbances in general) in electromagnetic fields (RF, light, etc...) always travel at the speed of light in that medium.
     
  5. BR-549

    Distinguished Member

    Sep 22, 2013
    3,163
    776
    You are almost 100% correct. But you need to qualify it. This will be getting into the weeds. A radio wave and a light wave is fundamentally the same thing. But their size has large effects. And not quite yet in a pure sense.......but in the future we should be able to transmit light from an antenna.

    Now that said.........one could induce a signal slower then light into an antenna. And with THAT said.....even though the induce speed is slower than light.......the field inducing it is not.

    It has to do with the different movement and structure........between an angular field.....and an emitted wave.

    In other words......we can not change the linear or straight path speed........but we can change the rotational speed.

    That's probably more confusing to you...........
     
  6. crutschow

    Expert

    Mar 14, 2008
    16,840
    4,586
    The speed of all electromagnetic waves (low frequency radio waves through light to gamma rays) is constant in a vacuum (and is nearly the same in air).
    One way is can be slowed is if it goes through a dielectric (such as glass for light), and even that slows it down only slightly.

    It's the frequency (and thus wavelength) of the electromagnetic wave that determines the type and size of the antenna, not its speed which is constant.
     
  7. MrAl

    Distinguished Member

    Jun 17, 2014
    3,916
    837
    Hi,

    If you look to the right, then to the left, then up, then down, you've just seen four different optical antennas. Almost all objects receive light and turn it into electrical currents. The catch is that the direction of these currents is somewhat random so there is no regular movement in any particular direction. This means we get heat but no usable electrical current unless the object is designed in such a way as to force a systematic movement that favors one direction.

    If we start with a wave of a certain frequency F traveling to the right and we look at the positive peaks and see one per second, we perceive that as 1 Hertz. If we speed up the movement alone to twice that speed, we will perceive that as 2 Hertz.
    A radio wave travels at the speed of light in a vacuum, but travels somewhat slower in different mediums. Most naturally occurring mediums only slow it down a little, but some man made mediums can slow it down to for example 38 miles per hour which is close to 18 million times slower than the speed of light in a vacuum. If we slow the speed of the wave down by 1/2, the perceived frequency will be half that of before because by the peak measurement method we'd see less peaks per second which is the frequency.
     
  8. BR-549

    Distinguished Member

    Sep 22, 2013
    3,163
    776
    They say that light slows down when going thru glass. Does the air on the other side speed it back up?
     
  9. wayneh

    Expert

    Sep 9, 2010
    13,746
    4,514
    Yes, it resumes its regular speed.
     
  10. BR-549

    Distinguished Member

    Sep 22, 2013
    3,163
    776
    What re-accelerates it?
     
  11. nsaspook

    AAC Fanatic!

    Aug 27, 2009
    3,771
    3,917
    cmartinez likes this.
  12. crutschow

    Expert

    Mar 14, 2008
    16,840
    4,586
    The speed of light is determined by the medium it is traveling through, but this difference in speed involves no change in the energy of the light quanta.
    Thus is travels slower when it goes through glass or water, and faster when it goes through air or vacuum.
    But that involves no change in the light energy, which is determined by its wavelength.
    So there is no "re-acceleration" as such, since this is no mass involved.

    It somewhat like a sound wave going through a solid or air.
    The sound speed will significantly increase in the solid and slow down when it reaches air.
    For example, the speed of sound in wood is about 4000m/s and in air is 343m/s.

    Of course the reason for the velocity changes are quite different for sound as compared to light.
     
    cmartinez likes this.
  13. MrAl

    Distinguished Member

    Jun 17, 2014
    3,916
    837
    Hi,

    Yes, look up the Doppler Effect.
     
  14. BR-549

    Distinguished Member

    Sep 22, 2013
    3,163
    776
    Maybe....because all media is composed of angular fields......some of the linear momentum of the wave.......is transferred to angular momentum to transverse the media. And upon leaving the media.....the proper phase and velocity is restored. In other words.......media reactance.
     
  15. Janis59

    Member

    Aug 21, 2017
    128
    24
    RE:""
    would the antenna be able to correctly get the frequency?
    I mean if it is looking for something travelling at lightspeed and the frequency picked up is slower, would it not count the peak to peak wave and then calculate it at being a frequency based on lightspeed""

    If I correctly translate the idea, You are worrying that in antenna METAL the radio-waves are becoming slower? YES, THEY ARE!!! But the difference is just few percents, larger in Al and smaller at Cu. Look for the precise values with querry `antenna shortening factor`. BUT the frequency !!A L L W A Y S!! is the same. Just if one miscalculates the antenna physical sizes forgetting to implement the shortening factor, antenna becomes be made slightly detuned, has bad gain at Your frequency and it gain is best for slightly lower frequency band as expected. In many cases this effect may be neglected, however sharp tuned antennas like Yagi-Uda the shortening factor is thing must-to-be taken in account.
     
Loading...