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Trouble with the idea of frequency.

  1. Jul 30, 2008 #1
    I've been looking around with google and I understand WHAT frequency is but I don't understand how it really works with light. For example, looking at a star if we are moving from it (or it's moving from us, or both) the light is red-shifted, but if we're moving towards it then it's blue-shifted. So, if I get in spaceship and fly towards this star at near the speed of light all I'll see is blue or something other than red right? But if I measure the speed of this light it'll still be c no matter if I'm moving from it, going towards it, or I'm standing still. How does that work? Are frequency and speed totally separate? Because it seems like they aren't when I look at all non-light examples on the net.
  2. jcsd
  3. Jul 30, 2008 #2
    1) Frequency is just how many times something happens per second.

    2) Waves look like this:


    So the distance (units of length) between two exact parts of a wave (i.e. two high-peaks) is called the "wavelength". Frequency is how many times per second the wave gets to its peak and can by found via frequency = velocity/wavelength

    Velocity in this case is how fast the wave itself is moving. Sound waves move faster or slower depending on what material they are moving through. They move faster in something like steel than air.

    3) You're talking about Special Relativity, which is an intermediate topic. The speed of light being the same no matter how fast you are going just *is*. That's how electromagnetism works. Now, that's a pretty crappy explanation, but that's all I can muster at the moment. Look up "relativity" to find out more.
  4. Jul 30, 2008 #3
    Thanks for the reply, a bit of a follow-up example.

    Say you have two lasers, blue and red, and you shoot them from point A to point B. They will both get there at the same time, but doesn't the blue light (because it is a higher frequency - more peaks) have to travel a longer distance than the red? If so doesn't that mean blue light travels faster than red?

    If you could freeze a wave/particle stream of light would it actually look like those waves or is that just a visual representation of some unseen aspect?

    Thanks, and sorry if these questions are just ignorant.
  5. Jul 30, 2008 #4


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    All light are electromagnetic waves and they all travel at the same speed, c. The blue light would "reach" point B the same time as the red one. You seem to be taking the picture of an oscillating particle as part of a transverse wave too literally. Note that that EM waves can propagate in vacuum, and does not require a medium. I think there was an experiment a few years back where some physicist froze light in its tracks, but I don't recall the details. But in the case of freezing a light wave, no you shouldn't expect to "see" the wavy sinusoidal waveform of light simply because unlike a transverse wave on a string, the amplitude of the waveform does not represent displacement of any medium but the magnitude of the electric and magnetic fields of the EM wave.
  6. Jul 31, 2008 #5
    No no no no no, you're confused about waves a bit. I know what you are thinking, that the particle travels along the wave, but that's not how it works. A photon travels straight. So you have a red photon and blue photon going at the same speed.

    Difference between them is the blue light is of higher energy.

    Well depends. If you were to freeze a sound wave, it would look pretty cool. A sound wave happens when you have a pressure difference traveling through a medium. So a sound wave in air would have parts of the air where air molecules are pretty close together, and other parts where they are further apart. It's kind of hard to explain without being there to draw something. =/

    Everybody starts somewhere. :wink:

    And you're trying to take in a lot of different concepts at once. I'd recommend looking up "waves", then "wave/particle duality", and then "special relativity".
  7. Jul 31, 2008 #6
    It also seems like you're thinking that the relative motion "filters out" the red light or the blue light - that's not right. The motion makes the frequency of ALL of the light more red or more blue; all of the frequencies are shifted over.
  8. Jul 31, 2008 #7
    Alright, things make a bit more sense now. Thanks everyone. I'll do a bit more research on the nature of light as well.
  9. Jul 31, 2008 #8
    This is a great topic. I struggle a lot of the keeping these ideas in perspective. What I am trying to say is, I can show mathematically how the physics works, yet, I lack experience(?) to explain to another person(s) exactly how things work, just as I am now!! egadz

    Of course, anyone can explain or show mathematically the result that's why we chose physics, right?(ignorance)

    I like reading everyone's posts,gives me insight. I just wish I could relate physics better from a more literal perspective.
    I am finding I have some serious issues concerning the factual understanding of physics. My question really centers around, "how do you retain all previous knowledge from prior course work and apply it to advanced work??" I know the answer is a trivial one, I just cannot seem to make it connect.


    Last edited: Jul 31, 2008
  10. Jul 31, 2008 #9
    Ok, well I will have plenty of practice this coming year for I will be taking a writing intensive course in E/M. What concerns me the most however, is my writing skills are suspect (uh-oh). Never had a writing intensive course till this year (yikes). this also another major concern I have, for when I take the GRE's, I fear I will crash and burn on my written test.

    It is hard, I have focused so much energy to disecting (spelling) problems and solving massive numbers of equations, I forgot what is pertinant!! If you cannot communicate your findings,then what damn good are you?? (just my critique of my own abilities, not yours).

  11. Jul 31, 2008 #10


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    If this helps any, I sometimes use the folowing analogy:

    Suppose I have several wind-up cars that all tavel at the same speed (2 ft/s). I set one down every 2 seconds, and they travel toward you across the floor. One will reach you every 2 seconds, and they will be 4 ft apart. Now, suppose I start walking toward you at 1 ft/s. In that case, one car will reach you every 1.5 seconds, and they will be 3 ft apart. But the cars are still moving at the same speed. In fact, it is precisely because they are moving at the same speed that this frequency change can be predicted.
  12. Aug 13, 2008 #11
    That's a good one LURCH, I don't remember hearing that before.
  13. Aug 13, 2008 #12
    Look up the Doppler effect, its basically what Lurch is describing. It works for every wave in classical mechanics but not in relativity. Light does not have a constant speed in classical mechanics either, this can be physically measured. What Lurch described is not true according to SR and is not the reason for the red shift in the universe so the Doppler affect does not apply according special relativity. My advice is get a good grasp on sound and other mechanical waves first. Once you get to special relativity, everything goes out the window.
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