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Frequencies of Light

  1. Mar 10, 2008 #1


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    My first post... I'm studying physics at school (9th grade) and learned that light has lots of different frequencies above/below visible spectrum. I also know that the speed of light is a constant (300Kps). My question is, does light (all frequencies) travel together like one multilayered wave... leaving and arriving at the same time? I just don't see how the light in the complete spectrum can travel at the same speed. Doesnt make sense. Is sound the same way? I know we have audible frequencies and inaudible. When a sound is struck, does all the sound from 1 Hz to 1MHZ (or whatever the highest is) all move together at the same speed?

    One final question. When I turn on a light bulb, does it have all of the frequencies, even though I only see certain wavelengths? Is the full spectrum always present in light, even ultraviolet and infrared?

    Sorry if my questions are too basic. I can't find the answers in my textbook, and I'm a bit introverted so I don't like to ask my teacher in class.

  2. jcsd
  3. Mar 10, 2008 #2


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    In normal media, all frequencies of sound and light have the same propagation velocity.

    In general, a light bulb will produce non-zero quantities of electromagnetic radiation all over the spectrum. It is very nearly a "blackbody" or thermal radiator, and the blackbody radiation curve does not fall to zero.


    Keep in mind that the vast majority of the energy is radiated in the infrared and visible parts of the spectrum. There's a little bit of ultraviolet, but that's stopped by the glass surrounding the filament. There's also some radio emission, but it's negligible.

    - Warren
  4. Mar 10, 2008 #3
    An interesting thing about sound: Suppose you're at a concert in a large auditorium. A person in the first row hears the sound of the tuba (low frequency) and the sound of the flute (high frequency) at the same time. A person in the back row also hear the sound of the tuba and the flute at the same time. Everyone in the whole place report that the musicians' timing was perfect. The speed of the wave didn't depend on the frequency. Imagine what would have happened if the speed of sound did depend on the frequency: in that case, it would only be possible to make the orchestra's timing sound perfect for the people in one part of the auditorium by messing it up for the people in another part of the auditorium. Luckily, we don't have that problem.
  5. Mar 10, 2008 #4
    There's a very slight dependence of wave speed on the frequency, but usually we won't notice it. But it's fun to mention a few places where we can notice it.

    When light crosses the boundary from air to glass, the light of the blue-violet end of the spectrum slows down just a tiny bit more than the light of the red-orange end of the spectrum slows down. Therefore the direction of the blue-violet rays bends more. This causes the spreading out (dispersion) of the colors that come out of a glass prism. I think the rainbow is the prettiest application of all.

    I'll tell you one place where this dispersion is a nuisance. When you're trying to focus a camera, movie projector, binoculars, etc., you can never focus it perfectly. One of the reasons for that is the focal length of the lens (the distance from the lens to a point, where parallel rays that went into the lens will come out of the lens to converge at the point) will be a slightly different length for the red end and the violet end. This is called chromatic aberration. We might never notice it, but it bugs the professional protographers.
  6. Mar 10, 2008 #5


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    Sodium lights

    Hi EVT! Welcome to PF! :smile:

    Here in England, a lot of our street light are sodium lights, which only emit two particular frequencies of yellow light.

    So my mum's yellow car, by comparison with everything else, used to look white at night! :smile:
  7. Mar 10, 2008 #6

    Claude Bile

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    Welcome to PF!

    Light in a vacuum will travel at the same speed, regardless of wavelength (c wouldn't be a constant otherwise). In transparent media such as air, water and glass, there tends is a very slight difference in speed with different wavelengths, because the refractive index tends to vary slightly with wavelength (the technical term for this is material dispersion) - and as you might be aware, light travels slower in media with higher refractive indicies.

  8. Mar 11, 2008 #7

    Andy Resnick

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    What you are asking about is 'dispersion'. Dispersion is the realtionship between frequency and wavelength. If it is a linear relationship ([tex]\nu=c_{0}\lambda[/tex]), as is the case for electromagnetic wave in a vacuum, the velocity of the wave is independent of the wavelength. In any material, the dispersion formula is given as [tex]\nu=\frac{c_{0}}{n(\lambda)}\lambda[/tex], where n(\lambda) is the refractive index. Now, the velocity is wavelength-dependent.

    Sound waves also have dispersion, becasue sound waves *must* travel in a medium.

    A lightbulb- specifically the tungsten filament, gives off a spectrum that approximates a 3000 K blackbody, and it has the majority of light output in the IR. The glass bulb and the air will modify the spectral ouput somewhat depending on the absorptive properties- think of colored christmas tree lights.
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