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

Is light faster in vacuum than it does in water?

  1. Mar 14, 2005 #1
    is light faster in vacuum than it does in water?????

    well first hi every one
    second i have a question?

    is light faster in vacuum than it does in water --->>> if so how you know the speed of light then (from equation...which make density of fluid in).

    ...and thank you
  2. jcsd
  3. Mar 14, 2005 #2


    User Avatar
    Gold Member

    Snell's Law
    http://www.ms.uky.edu/~skim/SpeedOfLight/ [Broken]
    Last edited by a moderator: May 1, 2017
  4. Mar 14, 2005 #3
    No. Light is always slower in water.

    Last edited: Mar 14, 2005
  5. Mar 15, 2005 #4


    User Avatar
    Science Advisor
    Homework Helper

    Let me see if I understand what you are asking:
    Is light faster in a vacuum than it is in water?
    If so, how do you know the speed of light (in water)?

    what you have in parenthesis isn't clear :confused:

    As pmb_phy mentioned and I agree, light is slower through water.. The reason is because; in a vacuum, light travels unimpeded and at constant speed (about 300,000 km/s). However through any other medium, light encounters atoms and molecules of that medium and becomes scattered, resulting in a slower speed.

    A good visual of this effect can be seen at --->

    For the question, how to measure the speed of light in water, cronxeh pointed out correctly to use Snell's Law. By observing how a beam of light passes through different media, you can measure an angle of incidence and an angle of refraction. Knowing the sines of these two angles, you can calculate the refractive index for water (1.33).

    It so happens that the ratio of speed of light in vacuum to speed of light in medium also defines the refractive index of the medium.
    You can then determine the speed of light through water by (300,000 km/s)/1.33 = 255,564 km/s

    An elaboration on this discussion with calcuations, can be seen at ---> http://id.mind.net/~zona/mstm/physics/light/rayOptics/refraction/snellsLaw/snellsLaw1.html [Broken]

    You can also determine the speed of light c through a medium, by measuring the permittivity [tex] \epsilon [/tex] and permeability [tex] \mu [/tex] of the medium; and using the relationship [tex] c = \frac {1}{\sqrt {\mu \epsilon}} [/tex] But then that is another story :wink:
    Last edited by a moderator: May 1, 2017
  6. Mar 16, 2005 #5
    well thank you all and cronexeh for the website now i've understood and its clear for me
    annd thank you all very much

    heaven eye
  7. Mar 16, 2005 #6
    qutache thank you very much
  8. Mar 16, 2005 #7
    am sorry but another question

    why dont scientists study light in solid be cause its much slower and know some of its describes to help them and help in relativity theory
  9. Mar 16, 2005 #8

    Andrew Mason

    User Avatar
    Science Advisor
    Homework Helper

    They do. Since an electron can actually move faster than the speed at which photons pass through water, light given off by a super luminal electron as it slows down appears as a bright burst similar to the sonic boom of a supersonic airplane (Cherenkov radiation). This is used, for example, in detecting neutrino-electron interactions in water (see how the Sudbury Neutrino Observatory uses Cherenkov light)

    [Actually, light itself does not travel slower in water than in a vacuum. Light always travels at c. But in passing through water, photons are being absorbed and re-emitted and this increases the time it takes for light energy to get through water.]

  10. Mar 16, 2005 #9
    That is incorrect. Light always has speed v equal to c

    That is true and this leads to the fact that it takes MORE time for a given energy-value (associated with the light) to pass through water. Moreover photons are also absorbed and emitted by the medium they pass through.

    However the speed of the photons is always c

    Untrue, read above

  11. Mar 30, 2005 #10
    well thank you all again but if the light speed is constant so what about light has speed in water about 200000 m/s and as we know light has speed of 300000 m/s so there is diffrent finally i proved that light speed are diffrent (or iam not understanding your points) !!!!!
    thank you all again may be iam making noise to you all here but iam so confused!!!!!!!!
  12. Mar 30, 2005 #11


    User Avatar
    Science Advisor
    Homework Helper

    That is an "apparent" velocity.It's accounted for by classical physics (classical electrodynamics).However,the processes of interaction of light with matter (e.g.water molecules) are studied in quantum context,where everyone knows (or at least,should) classical electrodynamics doesn't hold.

    But judging light as a wave (i.e.classically),yes,it can be said it travels at less than "c" in any material medium...

  13. Mar 30, 2005 #12
    Maybe im nto understanding you marlon but, your saying the speed of light is ALWAYS c? c being 3x10^8?
  14. Mar 30, 2005 #13


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Yes, he is.
    The speed of light appears to be slower in media because it is not continually propagating. Sort of like like driving down a street with a bunch of stop signs, You drive at 25mph, but your average speed is much lower, because you must stop at each intersection.
    Last edited: Apr 15, 2005
  15. Mar 30, 2005 #14

    Integral gave the best explanation of my point. These are the words that i should have written but couldn't find :rofl:


  16. Mar 30, 2005 #15


    User Avatar
    Gold Member

    Well that certainly clears up some loing-standing confusion for me.

    I'd always wondered why the central physics claim is that light moving at c is a universal constant that is never called into question, when we knew (I thought) perfectly well that it slows down all the time!

    But that description clears it up nicely.
  17. Mar 30, 2005 #16


    User Avatar
    Science Advisor
    Homework Helper

    That's why we invented quantum theories.To explain why light behaves like it does is one of the reasons...

  18. Mar 30, 2005 #17
    Is this also why chroot had me the other day, that the speed of light through medium can be passed but the absolute speed of light cant? The speed of light through medium is like a stop-and-go velocity, a phase velocity. The absolute speed of light is the wave velocity, right? The wave velocity is what cant be passed?
  19. Mar 31, 2005 #18
    well what i've just understood that 300000000 m/s is the speed of light not of photon while photon is kind of energy photon's speed can be change while light cant . therefore there is diffrent betweent light and photon

    >>>>>is that right!!!!!!!
  20. Mar 31, 2005 #19


    User Avatar
    Science Advisor
    Homework Helper

    No,no,light is made up of photons...Period.Photons travel of "c".Ergo light travels at "c" in any circumstance.

    Photons are particles.They carry energy & momentum.

  21. Apr 1, 2005 #20
    so what about shell's theory !!!!!!!!!!!!!!!!!!!!!

    thank you mr. dextercioby

    and i want to tell you that iam so beginner in physics thats why i asked (silly questions :blushing: ) be cause i dont know but funamental in physics
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook