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Monochromatic light

  1. Apr 29, 2011 #1


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    I'm doing work with a spectrometer and Hydrogen and Helium gas discharge tubes. Right now I'm studying the Balmer series for Hydrogen emission.

    Anyway, I'm trying to see what the "real" definition of monochromatic light is. Some references state it's light of one frequency while other references state that it's light of one wavelength? Which is it?

    If I look at a Hydrogen gas discharge tub, it looks "monochromatic" but when viewed through a spectrometer / diffraction grating, spectral lines at multiple wavelengths can be seen.

    How is monochromatic light made? Any examples of it in the physical world aside from in a lab?
  2. jcsd
  3. Apr 29, 2011 #2
    'Mono' = one; 'chroma' = color.
    How many wavelengths do you have for a single frequency of light?

    How does something 'look' monochromatic? If it has lines at multiple wavelengths, is it monochromatic?

    Real world example: LEDs and Lasers (although neither---especially lasers---are perfectly monochromatic, they have some spread/uncertainty).
  4. Apr 29, 2011 #3
    Monochromatic wave = single frequency.

    In the nature there is no monochromatic waves because emission of a light occurs at a defined fraction of time.
  5. Apr 29, 2011 #4


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    Every frequency has only 1 wavelength and vice versa. So saying that it is one frequency is exactly the same as saying it has one wavelength.

    In a discharge tube the multiple wavelengths of the light produces blend in with one another and show up as a single color in your eye.

    Normally monochromatic light is made by using lasers, LED's, or certain types of filtering techniques depending on the frequency you need and how close to the exact frequency you need most of the light. I don't believe there are any examples of pure monochromatic light in nature. But I'm not sure.
    Last edited: Apr 29, 2011
  6. Apr 30, 2011 #5


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    Okay that's intuitive... Thank you.

    A single color that is created by a single EM wave with multiple super-imposed wavelengths?

    I'm still not convinced there's real monochromatic light even in the lab. If it is possible, how does a laser create monochromatic light with exacly one wavelength?
  7. Apr 30, 2011 #6
    A true monochromatic wave only exists mathematically, and is usually represented as a simple sine or cosine function.

    In the lab, you can make nearly monochromatic beams with gas discharges, photodiodes, or various kinds of lasers, and add filters as needed. In each case, there is inherently a spread in the range of wavelength. The spread can be somewhat tuned, but cannot be absolutely zero. Examples of natural limitations are impurities of emitting material, non-zero temperature, and ultimately the uncertanty principle.
  8. Apr 30, 2011 #7


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    As far as I know, the narrowest bandwidth achievable for a continuous wave laser is ~100 Hz, and that is certainly far from "single frequency" in the context you are using it. There is a Fourier transform relationship between the temporal length of a laser pulse and the bandwidth. In order to achieve an infinitely narrow line, the temporal length of the pulse would have to be infinite.
  9. May 1, 2011 #8


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    Eyesight cannot see whether a particular source of light is monochromatic.

    Light that is detected by the light sensitive cells in the retina is processed. Our retinas do extract information, so we're sensitve to hue and so on, but the incoming light is not resolved in constituent frequencies as happens with a diffraction grating.

    For most purposes the light of a Sodium lamp is effectively monochromatic. But a sufficiently high spec diffraction grating shows that Sodium lamp light is two spectral lines that are very close together.

    When a light source is referred to as 'monochromatic' what is meant is that the light is sufficiently monochromatic for the intended purpose.
  10. May 1, 2011 #9


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    This is what I suspected... That a true monochromatic wave doesn't really exist. I'm sure that certain lab setups are "close enough" so we can model the light as being monochromatic.

    I've heard the word "spread" a couple times now. What does it mean when you say that light from a nearly-mono-chromatic laser has a spread?

    I know that the uncertainty principal states that you cannot know the exact momentum of a particle because observing it actually changes its trajectory. How does that factor into the fact that light can't be 100% monochromatic?
  11. May 1, 2011 #10
    1. If you could measure the wavelength of a (sufficiently) monochromatic beam on a short enough time scale, the value would be slightly different each time. Drawing this on a graph, you would get a bell-shaped curve. The spread is the width of the curve.

    Otherwise, pass a beam in a prism. A polychromatic (such as white) beam will spread in the rainbow colors. Well, a seemingly monochromatic beam will also spread, ever so slightly.

    2. p = hf (a photon's momentum and wavelength are related by this very simple relation, h is a constant (Planck's)). This is correct but both p and f always have an uncertainty, or a "spread", or a linewidth, or an error (all practically synonyms in this context).
    Last edited: May 1, 2011
  12. May 1, 2011 #11

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    It's no different than frictionless planes, stretchless ropes, massless pulleys, etc. If you want to make progress, you have to make approximations.

    Truly monochromatic light takes an eternity to determine that it is so. So there is no physical source that is perfectly monochromatic. However, many sources are very, very close, and as such it is a useful abstraction. Just like the stretchless rope.
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