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Wavelength of light in relation to speed

  1. Apr 22, 2015 #1
    Hi guys, first ever post.

    My question is if light travels at the same speed regardless of its wavelength, doesnt this statement seem to contradict itself?

    If the light simply travelled in a straight line then sure, but different wavelengths i would think mean that a greater distance would have to be travelled, if it was going the same speed in one direction as light with a shorter wavelength. Ofcourse maybe i have misinterpreted something as i am not super educated on the subject (I have a HNC in eletrical engineering) and obviously light has all sorts of properties that are unique to it. Is there a flaw in my question or is there something else at play?

    Very good forum btw everyone!
     
  2. jcsd
  3. Apr 22, 2015 #2
    Sound waves always travel at the same speed (~300 m/s), and they have different wavelengths (pitches) as well.
     
  4. Apr 22, 2015 #3
    but if the wavelength is longer, doesnt the sound or light have to travel a greater distance along the wave than one would with a smaller wavelength?
     
  5. Apr 22, 2015 #4
    Nope, it just wriggles faster or slower. The wave itself, be it sound or light, proceeds at the same speed, no matter how fast it wriggles.
     
  6. Apr 22, 2015 #5
    thanks rumborak. so the wavelength size has an effect on the frequency. and the two combined always equal the speed of sound/light. so one increases while the other decreases. i never knew they were related in that way, makes sense though!
     
  7. Apr 22, 2015 #6

    davenn

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    yup

    wavelength is inversely proportional to frequency ..... shorter wavelength higher freq and visa versa
     
  8. Apr 23, 2015 #7
    thanks, one final point is there's one other factor in the wave itself which is the amplitude. is there any set proportionality betwee amplitude and the other 2 factors, as if the speed is fixed i would guess that it implies that it would be, given that it is part of the distance that the wave travels.
     
  9. Apr 23, 2015 #8

    Drakkith

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    You seem to have made a distinction between 'light' or 'sound' and 'the wave itself'. There is no distinction. Light and sound are the waves. They don't travel along the wave, as a roller coaster cart moves along the rails of the track, they literally are the wave.

    A sound wave is a compression wave. That means that an volume of air is compressed while right behind/in front of this compressed volume there is another volume of air that is rarefied. See the following animation:

    260px-Spherical_pressure_waves.gif

    You can imagine that the white circle compresses the air (the dark rings), and this circular volume of compressed air moves outwards from the source, leaving behind a volume of rarefied air, which is less dense than average. When this compression wave hits your eardrum it exerts pressure on it, causing to vibrate and initiating the hearing process.

    A light wave (otherwise known as an electromagnetic wave, or EM wave) is much harder to visualize since we are talking about oscillations in the electric and magnetic components of the EM field. Put simply, an EM wave is an oscillation in the force exerted by the field on a charged particle. In other words, if you look at the charges in an antenna as an EM wave passes by, they will be accelerated in one direction, then stop, and then be accelerated in another direction before stopping and having the whole cycle repeat again. The wavy graph you see when you look up anything on light is exactly that. A graph. Light is not two sine waves set perpendicular to each other. Those sine waves just represent the direction and magnitude of the forces exerted by the field.

    When talking about the speed of light, we are talking about how fast the wavefront propagates, not how quickly the fields oscillate. Since the oscillations in the field are not physical motions but changes in the force exerted by the field, the idea of 'speed' doesn't apply to them. Instead, the oscillations are measured purely in cycle per unit of time, usually cycles per second, otherwise known as hertz.

    In free space there is no connection between the frequency or amplitude of the wave and the speed. However this isn't the case when light travels through a medium. For example, glass will slow light with a lower frequency less than light with a higher frequency, which gives rise to dispersion.
     
    Last edited: Apr 23, 2015
  10. Apr 23, 2015 #9
    Thanks Drakkif. If i'm understanding you correctly you're saying that you should not relate the speed of light to the oscillations of the wave, which have no effect on it as they are exerted by the force from the field? I thought that with frequency increasing as wavelength is decreasing, and vice/versa gave rise to the constant that is the speed of light. But what you are saying is that amplitude, wavelength and frequency are just properties of the energy of the photon and not related to its speed. Is that true?

    I must admit I'm although i'm fascinated by particle physics and quantum phenomena it seems i can't thoroughly understand some basic stuff, or maybe i'm just applying the rules of the macro world too much (I know at subatomic levels things are different) but it just seemed to make sense that the larger the wave's characteristics, the more distance that was being travelled if measured from a set point in a straight line. Have a look at the daigram below:
    Untitled.jpg
    Basically the original question was asking that in some cases would the distance from A to B, be longer in some waves than others, and if so would that mean that over a set linear distance in length, that there would be more 'wave distance to travel', in effect the total distance travelled being greater, although in ilnear speed they move the same speed, which would suggest the light has to move faster. Thanks for the in depth answer by the way Dakkrah, here I'm merely trying to error proof any confused thinking i still have!
     
  11. Apr 23, 2015 #10

    jbriggs444

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    The amplitude of a light wave (forget about photons) is not a distance. It is a field strength. The light wave is not following a squiggly path through space. It is going straight and is modelled as a self-propagating change in field strength. The graphs you show above would be graphs of field strength versus time, not of transverse position versus time.
     
  12. Apr 23, 2015 #11
    Thanks jbriggs444, I always thought of the wave as a literal representation of the light through space, as frequency is after all cycles per second and the amplitude i always thought of as a physical 'length' rather than 'strength'.
     
  13. Apr 23, 2015 #12

    davenn

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    no, there is no relationship between the amplitude and the freq/wavelength

    examples ...
    1) a high power radio transmitter and a low power one on the same frequency
    2) the light output from a small flashlight globe and the output from a big spotlight

    Dave
     
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