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What do we mean by 'wavelength'

  1. Jan 8, 2010 #1
    hi everybody

    In sound waves or EM waves ..
    What do we mean by saying 'wavelength' ! physically
    I mean ,
    Is that related to the size of the photon for example ( In EM ) ?
    Help me to understand
  2. jcsd
  3. Jan 8, 2010 #2


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    Hi MiniSmSm! :smile:

    Light is an EM wave, a disturbance in an EM field.

    The wavelength is the distance that it takes for the disturbance to repeat itself.

    It's not related to the size of a photon (whatever that is :confused:), but it is inversely proportional to the momentum of a photon (and to the energy).
  4. Jan 8, 2010 #3
    :) Thanx tiny-tim I appreciate that ..
    but how could we say that is related to the color of wave ?
    plz explain..
  5. Jan 8, 2010 #4
    Do you mean visible color? Essentially your eyes takes in the light (and the wavelength associated to the light) and your brain interprets the wave as a color.

    The typical human eye can see light that is between the wavelengths of 400nanometers to 700 nanometers. 400nm is roughly violet and 700 roughly being red. The colors in between are roughly the colors of a rainbow in 50nm intervals. So something like:

    Violet 400-450nm
    Blue 450-500nm
    Green 500-550nm
    Yellow 550-600nm
    Orange 600-650nm
    Red 650-700nm

    Right below violet in the light spectrum is ultraviolet (which we cant see with the naked eye). Right above red is infared (which we also cant see with the naked eye).

    A shorter wavelength means a larger energy as tiny-tim touched upon. So violet is the most energetic color that is visible to us naturally, while red is the least energetic.
  6. Jan 8, 2010 #5


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    Red light is hotter (it has more energy, and more momentum) than blue light.

    Colour depends on frequency (1/wavelength), which is proportional to both momentum and energy. :smile:
  7. Jan 8, 2010 #6
    Not sure about your first point but the energy of light is inversely proportional to its wave length. Or, in other words a smaller wavelength means that the light is more energetic.

    As far as heat goes. Red is the coolest you can heat an object and visibly see it. This is why you know a fire is really hot if it's got a hint of blue/violet to it.
  8. Jan 8, 2010 #7

    Red Light is hotter !! or blue light is hotter than red ?
    blue light has more energy than red light
    so how can red light be hotter than blue ?
  9. Jan 8, 2010 #8


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    oops! :redface: wrong way round!
  10. Jan 8, 2010 #9
    Things that glow red are colder than things that glow blue.

    Temperature and energy are directly related so something that is more energetic (blue light) is hotter.

    Blue light - hotter and more energetic
    Red light - cooler and less energetic
  11. Jan 8, 2010 #10
    ThanX for clarification :)
    Last edited: Jan 8, 2010
  12. Jan 8, 2010 #11
    The basic way to correlate color and wavelength is to use a ruled reflection (diffraction) grating, and measure the diffraction angle, hence wavelength, for each color. See
    http://www.edmundoptics.com/onlinecatalog/displayproduct.cfm?productID=1896 [Broken]
    Bob S
    Last edited by a moderator: May 4, 2017
  13. Jan 8, 2010 #12


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    If you mean, "what is the mechanism by which we perceive different wavelengths as having different colors?" that is a matter of biophysics (how the receptor cells in the eye respond to different wavelengths) and neurology (how the brain processes the information that it receives from the eye). You'd probably better ask about that in a biology forum, maybe even the one here on PF.
  14. Jan 10, 2010 #13
    ok I know that blue light has more energy then red . I believe you . but why is an alcohol flame that is blue colder than a red flame made form a fire with pine wood.
  15. Jan 10, 2010 #14

    Vanadium 50

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    Why do you think an alcohol flame is colder?
  16. Jan 10, 2010 #15
    becuase the chemical reaction produces less heat . i cant really take it any deeper than that
  17. Jan 10, 2010 #16


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    That would appear http://en.wikipedia.org/wiki/Ethanol#As_a_fuel". Look at where wood is on the chart.

    In fact, I wonder if that's related to the fact that there are lots of alcohol-burning vehicles there are out there, but not so much wood-burning vehicles...
    Last edited by a moderator: Apr 24, 2017
  18. Jan 10, 2010 #17
    ok I see , but there was another thread about the color of flames and there energy that addressed this question , because gasoline burns red orange and it burns hotter than alcohol .
  19. Jan 13, 2010 #18
    well , thanx ..
    that`s really what I meant
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