Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Question regarding lightspeed

  1. May 19, 2005 #1
    Is lightspeed defining the speed of light in which it travels a vertical length?
    Can lightspeed be used to accurately decide how long it will take for light to travel 5 meters straight vertically?

    Considering that light has wave length that 'should' not be the case.
    Does this mean that different types of light travels faster than others in a straight direction?

    Hopefull for answers, tell me if I need to clarify what I mean better.

  2. jcsd
  3. May 19, 2005 #2
    I also want to know if light or anything for that matter has wave lenghts in vacuum.
  4. May 19, 2005 #3
    Yes padawan. To clarify better we want you.

  5. May 19, 2005 #4
    Well, your second post... yes. Light travels in a vacuum and can be described as a wave with a wavelength. I dodn't understand your first post, sorry. The direction the light travels in is irrelevant, in fact arbitrary. Yes, you can tell how long it will take for light to travel 5 m.
  6. May 19, 2005 #5
    Since light has a wave length it doesn't travel in a straight line.
    Hence I was wondering wether lightspeed is the definition of the time it takes for light to travel a vertical length or if the wave lengths are accounted for aswell.
    I'll try to make a picture..:)

    ^ ^
    / \_/ \

    Is lightspeed the speed at which the particle is moving? In that case wave lenght would speed down the vertical speed.
    If not, then lightspeed < the speed at which the particle is moving.

    Hope that helped.

    EDIT: (The picture changes appearence after I hit the 'post' button, another mystery of the universe)
  7. May 19, 2005 #6


    User Avatar
    Science Advisor
    Homework Helper

    It helped in making the confusion even larger.The light in vacuo,treated as a classical wave,has the nice property of non dispersion,which translates into phase velocity is equal to group velocity and equal to "c".If u know the definitions for the two kinds of velocity,then u can simply get the exact meaning of "c".

    HINT:since v_{g}=c,it won't be so obvious.I suggest you pick the mathematical form of harmonical scalar em wave travelling in vacuum:

    [tex] \psi (\vec{r},t)=\psi_{0}e^{i\left(\vec{k}\cdot\vec{r}-\omega t\right)} [/tex]

  8. May 19, 2005 #7
    I'll give this one last shot before I give up.
    I'm sorry but I didn't understand half of what you just said Dextercioby, thanks for trying though.

    Say that you are driving from New York to Los angeles´, it's 500miles bird route but since you have to follow the road it's 750 miles (I'm just making these numbers up).
    This can be compared to light following it's road (going in waves).
    5meters 'bird route' might be 7meters considering that it follows a road (not going in a straight line but in waves).

    With that in mind, is lightspeed the speed at which light covers the bird route?
    Since different lights has different wave lenghts the speed at which they cover the bird route would be different.

    Hope this didn't leave you even more confused.
  9. May 19, 2005 #8


    User Avatar
    Science Advisor
    Homework Helper

    Nope,a pointlike light source produces a perfect spherical wave whose surface of constant phase propagates at "c" in vacuum.It's the simplest explanation of light propagation one could give and very useful for special relativity.

  10. May 19, 2005 #9
    Er, young padawan, you are saying strange things. Beware, you are closer to the dark side of the force.

    I will give you an analogy. Take 2 balls flying at the same speed in a bird route. Now say that the ball is a light of a given wavelenght (the size of the ball). Why do you want to get a different speed for a different size (a different wavelenght? (they are both equal in this example).

    I hope this may help you, young padawan, staying ahead of the dark side of the force.


    Daniel, to what side of the force do you belong?
    Last edited: May 19, 2005
  11. May 19, 2005 #10
    Tage, I believe I understood the source of your worry. The light itself does not move up to its crest and down to its trough as it propogates. These amplitudes relate to the waves energy density. It is not the path the light actually travels by.
  12. May 19, 2005 #11


    User Avatar
    Staff Emeritus
    Science Advisor

    You got a completely wrong idea! Light "waves" are changes in the electro-magnetic field not in space- light does, indeed, travel in a straight line.
  13. May 19, 2005 #12
    Thanks everybody for clarifying, I finally got it :)
  14. May 19, 2005 #13
    Hourrah, young padawan !
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?