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How Wavelength is measured?

  1. Sep 27, 2010 #1
    How the wavelength of wave is measured?
    My idea of electromagnetic wave is there is an invisible line traveling through space moving up and down like a sine wave we see on CRO. Is this idea correct? Or is it the the variation in electric and magnetic field of the wave that we relate this wavelength with and there is no such invisible line?

    Please explain. I have searched over net for electromagnetic wave, all pics shows two sine waves(representing magnetic field and electric field) perpendicular to each other. Is this how the wave would look like if we can see it?
  2. jcsd
  3. Sep 27, 2010 #2
    Sine wave is easy to draw and understand! However the real wave looks like several arrows and sine wave can be formed from the tip of arrow head
    See this site http://en.wikipedia.org/wiki/Electromagnetic_radiation
    There is a picture with blue and red arrows, which represent the true wave. If you know the speed or energy, you can calculate the wave length.
    Hope this helps.
  4. Sep 27, 2010 #3
    The light is oscillating electric and magnetic fields, if it is continuous wave you can imagine it like this, if it's pulses of light then you have other ways of thinking of it (group velocities and envelopes), if it's X-ray or gamma ray then you're best thinking of it as individual photons.

    Different frequencies/energies interact differently with matter and that's essentially how we see the wavelength. Cameras have photosensetive film or CCDs/CMOS sensors which convert light into electric pulses, etc. Your final question is very interesting because we can see the wave! Your eyes do a similar thing- detecting light at certain frequencies and build it into an image using a lens and some brain power.
  5. Sep 27, 2010 #4

    Andy Resnick

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    Good question. Optically, spectroscopic measurements can measure the wavelength to a very accuracy by using diffraction to convert the wavelength into an angle (which is basically what inverse wavelength means):

    http://www.nist.gov/physlab/div842/grp01/highreso.cfm [Broken]

    Acoustically, I think the frequency can be directly (coherently) measured, and the wavelength computed:

    Last edited by a moderator: May 4, 2017
  6. Sep 27, 2010 #5


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    Correct :smile:
  7. Sep 27, 2010 #6
  8. Sep 29, 2010 #7
    Thanks for the replies!!
    The image below might help to explain my imagination

    In above image, suppose there is transmitter on left side and receiver on the right. In between is the wave. Is this how the EM waves travels in air? If we could then we would see a line in air connecting transmitter to receiver. and that's how we could measure its frequency and wavelength, as shown in above image. Is this correct?

    Or the frequency of the wave is the variation in the magnetic and electric field?
    Last edited by a moderator: Apr 25, 2017
  9. Sep 29, 2010 #8


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    Well, I did answer this already in Post #5, but here it is again.
    That is not what we see. It is not what an electromagnetic wave is.
    Yes, that is exactly what the wave is. It is a variation in the electromagnetic field. It is not a squiggly line traveling through space.

    Hope that clarifies it for you :smile:
  10. Sep 29, 2010 #9


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    Yes. See the diagram in the post linked below for a somewhat more accurate representation.

  11. Sep 29, 2010 #10


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    In order to start off on the correct footing I should point point out that the dimension shown in your diagram is actually twice the amplitude. If you take a sine wave of amplitude A then it peak -to peak value will be Asin(90)-Asin(270), which is 2A.

    Your picture is just a graph of the value of, say, the Electric field, as it varies with time and space. There is no 'line' anywhere, any more than there would be a sloping line between the North Pole and the Equator just because the temperature increases over that distance.

    You can 'see' waves on water and they do, actually have a shape to them but that's because something (the surface of the water) is actually moving from side to side. Not so with electromagnetic waves which propagate even when all there is is empty space - it's just Fields varying.

    In space, the disturbances of the fields (waves) will be at right angles to a line joining transmitter to the receiver. This 'rectilinear propagation' was one of the very early observations about light - long before it was found to have a wavelike nature. The path can be bent (refracted) when the nature of any medium it is passing through, is changing but, at any instant, the fields are still at right angles to the line of propagation.
    Last edited by a moderator: May 5, 2017
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