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

How fast is light on Earth?

  1. Oct 17, 2005 #1
    When light enters in Earth's atmosphere, how fast does it go? Or does it change at all?
     
  2. jcsd
  3. Oct 17, 2005 #2

    Danger

    User Avatar
    Gold Member

    The speed of individual photons doesn't change, but the propogation rate of the 'beam' does so in accordance with the composition and density of the air that it's passing through. That varies.
     
  4. Oct 17, 2005 #3

    How much does it change on the average in accordance of Earth's atmosphere?
     
  5. Oct 17, 2005 #4
    not much. the speed of light in air at NTP is nearly the same as in vacuum and differs even less at lower air density.
     
  6. Oct 17, 2005 #5

    Danger

    User Avatar
    Gold Member

    That's beyond my knowledge, but it's a really tiny percentage of c in vacuum. Time for someone else to take over here.

    edit: Ooops. I took a phone call between typing and posting. Didn't know you were here, inha.
     
  7. Oct 17, 2005 #6
    Well, I think you need to be a little more precise on this subject, Danger. As far as I can tell the light beam (you're referring to) is made up of individual photons, isn't it? So what do you mean the individual photons don't change speed?
    @randy23: In principle you could just apply the laws of optics to this problem, as the phenomenon of light passing from one medium to another has been studied very well. The problem is that it is fairly hard to calculate what is going to happen precisely, as that depends, as Danger already stated, very much on the "composition and density of the air that it's passing through". Now, if you simplify things (uniform air), you end up with very much simpler calculations, but that's not what you wanted, right? So you won't get an answer, I'm sorry to tell you, but that's the way it is. You need to content yourself with the fact that the speed of light doesn't change substantially.
    Regards...Cliowa
     
  8. Oct 17, 2005 #7

    Danger

    User Avatar
    Gold Member

    The photons themselves travel at standard c over inter-atomic distances. It's the absorbtion and re-emission thereof by the atoms that slows down the overall propogation.
     
  9. Oct 17, 2005 #8
    Alright, thanks. Just wanted to know what exactly you're refering to.
     
  10. Oct 17, 2005 #9

    Integral

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    While the changes are small, they are observable. The most obvious effect are road mirages. The INDEX OF REFRACTION of air is dependent on density, which in turn is dependent upon temperature. There is a well developed temperature gradient near the surface of a paved road. It is this variable density gradient which causes road mirages.
    You might try seareching on "index of refraction of air"
     
  11. Oct 17, 2005 #10
    Danger, how would you describe light propagation is glass then?
    The inter-atomic distances would be much smaller than the wavelength of visible light.
    And it would be still much smaller compared to microwave wavelengths.
    Still, microwaves are refracted by glass or by a lot of materials (like ceramics).
    The classical view seems easy to me. But the quantum picture seems 'unspeakable'.
    Any helps ?
     
  12. Oct 17, 2005 #11
    randy23, the reason that Integral suggested this search is that the index of refraction tells you how much light slows down in a given medium with respect to its speed in vacuum.

    Here is a site that has the index of refraction for various substances.

    http://www.is.kiruna.se/~cjo/d2i/REFRACTION.INDEX3.html

    Note that for air, the letters STP stand for "standard temperature and pressure" and for air of differing properties, the speed will be different.

    The way to calculate the speed of light in a given medium is to divide the speed of light in vacuum (299,792,458 meters per second) by the refractive index for that medium (in your case 1.00029). In your case I get roughly 299706000 meters per second. However, be aware that as light first hits the atmosphere from the vacuum of space, stp conditions do not apply.

    Given this interpretation of index of refraction, it makes sense that for vacuum, the index is 1.
     
  13. Oct 17, 2005 #12
    Kudos for the help everybody.
     
  14. Oct 17, 2005 #13

    Danger

    User Avatar
    Gold Member

    I'm afraid that's outside of my knowledge. I've never had much of a chance to study QM. One thing about it is that the photon isn't necessarily absorbed by a directly-neighbouring atom, so the uninterrupted propogation length would be variable. I'm not familiar with the exact QM interactions between electrons and photons.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?