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Thread: Speed of light through different mediums

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    Default Speed of light through different mediums



    Quote Originally Posted by Learjet View Post
    Bizarre.

    "The speed of light, as we've all heard, is a constant: 186,171 miles per second in a vacuum. But it is different in the real world, outside a vacuum; for instance, light not only bends but also slows ever so slightly when it passes through glass or water. Still, that's nothing compared with what happens when Hau shines a laser beam of light into a BEC: it's like hurling a baseball into a pillow. "First, we got the speed down to that of a bicycle," Hau says. "Now it's at a crawl, and we can actually stop it—keep light bottled up entirely inside the BEC, look at it, play with it and then release it when we're ready."
    Evening all

    After reading Learjet's above reply from the "coldest place in the universe" topic, it got me wondering... If the speed of light slows as it passes through a glass of water how come we don't see it as a different color? or does it regain it's momentum after departing the water? Or have I just misread what was being said and the speed of light doesn't change, or just completely screwed up my thoughts??

    Cheers guys and gals



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    Quote Originally Posted by Gitch View Post
    If the speed of light slows as it passes through a glass of water how come we don't see it as a different color? or does it regain it's momentum after departing the water? Or have I just misread what was being said and the speed of light doesn't change, or just completely screwed up my thoughts??
    It does come out different colours.... have you noticed the rainbow on your wall when you put a glass of water on the window ledge.
    And this directly caused by the difference in velocity of light from water to glass/air.

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    The speed of (light) electromagnetic radiation is constant and absolute and referenced to a vacuum.

    The speed at which light itself travels through different mediums changes.
    Light travels slower through denser mediums. It travels slower through air than it does a vacuum. Slower again through glass than air, or water than air or glass.

    If a wave is traveling at a particular incidence to the different medium, one side of the wave front is now traveling slower than the other and like a car with half its wheels on the road and the other half in the gravel, the car's path becomes bent in a similar way.

    In the case of our wave traveling slower through the denser medium it will appear to have a different wavelength or frequency. But one has to observe the light from inside the medium to notice this. If in our case, the light exits the glass and now travels through like to our eye at the higher speed, we will see the light as it was before it entered the glass.

    This happens to all kinds of eletromagnetic radiation, and even radio or electronic signals in cables. Some frequencies travel at different speeds through the cable or optic fibre and waveguides.
    The effect is called group delay and it can "smeer" high speed data.

    It telescopes it causes some unpleasent effects with the colour of stars.
    There are methods taken to correct it. In the case of telephone signals or radio, equalisers are placed in the circuit.

    In some unusual cases, some forms of electromagnetic radiation are travelling faster than light through a particular medium. An example is Gamma Rays traveling in water. Gamma Rays are slowed down as they travel through water, but light itself is slowed down even more. Though both are not traveling faster than the absolute speed of light, the gamma ray does appear to be traveling faster than visible light. The effect is that it creates a light shockwave like a plane flying faster than the speed of sound.
    The gamma ray leaves a blue glow in its wake as it passes through the water. This blue light is called Cherenkov Radiation.
    The glow is quite visible when Cobalt 60 is placed in water.
    The effect occurs in lots of places including in air, though it is not usually visible with the naked eye. A cosmic ray passing though air will cause the air to glow, but it can't be seen.
    A scintillator (a type of geiger counter) has a crystal in it and a light sensor.
    When a particle passes through the crystal, it glows for the same reason and the sensor counts the light flashes.
    On a big scale, neutrino observatories are also just bug scintillators. A neutrino passing through the water will cause a very weak flash. The sensors look carefully at the flash and measure its speed and direction.
    If neutrinos were actually traveling faster than the absolute speed of light, the neutrino dectectors would be able to measure their speed.

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    Thanks Trash

    That makes sense now, not to mention the fact for some reason at the time of writing my question I was thinking along the lines of light being particles and as such would require energy to increse their speed after exiting the denser matter. Dumb I know, I think I was tired :P

    But cheers for clearing that up, my life can now resume normallity

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    Quote Originally Posted by Gitch View Post
    Thanks Trash

    That makes sense now, not to mention the fact for some reason at the time of writing my question I was thinking along the lines of light being particles and as such would require energy to increse their speed after exiting the denser matter. Dumb I know, I think I was tired :P

    But cheers for clearing that up, my life can now resume normallity
    Doesnt electromagnetic radiation exibit both wave and particle charactristics?

    Isnt the light particle called a Photon?

    Is RF EMR also made up of Photons?

    My understanding was that Photons are massless particles?

    Anyway....

    porkop

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    It's correct to think of EMR to be made of particles.
    It's also correct to think of them in terms of waves.
    The exhibit properties of both.

    It is possible to define the same characteristics in terms of particles.
    My head is a bit fuzzzzzy at the moment as I'm entertaining a member of the opiate family. I'm trying to recall the nature of the duality of particles and waves. We can define the postion of a particle, but not its motion at any point in time. Likewise you can define the motion of a wave, but never its precise location.

    Anyhow ... I was taught at high school that the rest mass of a photon was zero. And the volume of an electron was also zero. I upset my physics teachers class when I questioned him in class and gave the example of why the rest mass of a photon was not zero. I was assured it was zero, and that I shouldn't rock the boat.

    I'm not sure if it was just a simplification of physics for high school or that was the actual 1980's scientific knowledge. When I read back over old 1800-1900's scientific texts, I'm absolutely amazed at how they deduced some theories. It was probably a simplification.

    There are plenty of tests that can be conducted, I like the use of gravity. Since gravity is a force, mass x accelleration, and light can be accellerated by gravity, it must have a mass.
    But photons travel very close to the speed of light, and so a large part of their mass will be relatistic.
    If we use E=mc^2 ... actually E=mc^2/SQRT(1-(v^2)/(c^2))
    we can see that if m=0 then under all circumstances E must also be zero.
    Which would mean if light had no rest mass, it would also have no energy no matter how big v was.

    Since I don't have any of my particle physics books at hand... I just decided to look at wikipedia. And I think they've got it pretty well covered.

    They've got the wave particle duality covered, they have the bose-einstein thing covered for tytower.
    I can see they made mention of the rest mass of an photon in terms of electron volts. It makes some sense, but I find I can often get confused with using this term for mass. I like kilograms.
    The rest mass of a photon is guestimated as 1.1×10−52 kg (6x10-17 eV), respectively

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    Hi every1,

    New to the forum...interesting topic.

    Actually this is a real furphy even in the physics teaching world.

    The speed of light is NEVER different to the vacuum speed (3 x 10^8 m/s).

    What happens in other substances, (say water), is that the photons hit atoms and are absorbed to move electrons into a higher energy level. This takes a very small amount of time.
    Then the electrons drop back and reemit a photon of light which moves on...and so the process goes.

    The time taken for all these absorptions and reemissions is very small but not negligible. Hence the TIME it takes for *light* to travel through water is longer than for the same trip through air.

    It is safest to say that the *average velocity of the light* is lower...but in reality...the trips between atoms are always at the usual vacuum speed (3 x 10^8 m/s).

    Light is ALWAYS interesting!

    Cheers,

    Kevin from Wycheproof.

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    Mmm, it's slower but it isn't.

    Interesting stuff, however I'm starting to feel like Colonel Jack O'neal from Stargate SG-1 when it comes to this stuff now Lol.

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    The speed of light when traveling through air is roughly 299,460 km per second. When light passes from air into a denser medium at an angle, like glass or water, its speed slows down by the index of refraction of the medium.

    Some examples of different mediums and their respective index;

    Air/Vacuum with an index of 1.0 which is 299,460 km/sec
    Water with an index of 1.33 which is 225,158 km/sec
    Glass with an index of 1.5 which is 199,640 km/sec
    Diamond with an index of 2.42 which is 123,744 km/sec

    As the wave of propagation is still continuous, this slowing down bends the light beam when it enters the new medium. It is similar to a bicycle changing direction when it enters sand from road.

    Just thought i'd add my 2c.

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    Quote Originally Posted by CaptainZero View Post
    What happens in other substances, (say water), is that the photons hit atoms and are absorbed to move electrons into a higher energy level. This takes a very small amount of time.
    Then the electrons drop back and reemit a photon of light which moves on...and so the process goes.
    Got ya umbrella, time to rain on this parade.

    There is a LOT of space between atoms. So I would expect to see light smeered if only some photons were slowed down, while the majority continued on their merry way at full speed. Group delay should be measurable in medium if that was the case, not including normal scattering.

    Next is that when an photon is absorbed, the light that is emitted from the same atom can only be emitted at specific energy levels. Which reminds me... I must check a glass of water for Hydrogen and Oxygen absorbsion lines.

    Next example is a wave traveling in the same medium, air, but traveling at different velocities over water or land. Most of the hamd here will be able to demonstrate an example of Radiowaves refracting over a coastline.

    That's low frequencies, then we have a few more examples of where the atom absorbing theory is a little questionable.
    Up at X-rays we have crystal diffraction, the atom involved can only re-emit lower energy quanta and a material either fluoresces or it emits very specific K bands.

    The big giveaway with all this is; How does the excited atom remember which way to emit the photon ? Light or EMR should be scattered in all directions by constant absorbtion and emission.

    Next question ... the density of different gasses. If it is the atoms themselves slowing down the light, do different atoms slow light down at different rates ? Does a heavier gas like SF6 refract light more or less than the surround air at the same temperature and pressure ? There are after all, the same number of molecules in the same volume of gas.

    Here's a nasty one. If it is the atoms effecting light, why is it whole molecules effect the way light behaves ?
    Carbon Monoxide, and Carbon Dioxide are both colourless gases, very similiar in nature to each other, yet they have different spectra and refraction indexes at the same temperature and pressure. A few less oxygen atoms, might just throw some extra O2 in there to attempt to even it up. Opps ! That don't work either ? It makes it worse.
    Different Alotropes of Carbon, Graphite and Diamond. Slightly different light conducting properties there for nothing more than a difference in chemical bonding.

    The final nail in the coffin is different vacuums, where there is more than a lot of space between atoms. And for a visibile amount of light we are talking trillions upon trillions of photons per second for nowwhere near as many atoms.
    Even when we raise the pressure and introduce more atoms, the chances are most of the photons are going to pass through the medium without ever meeting up with an atom. By the logic you mentioned, these first atoms should be arriving at the target at the same time in a variety of different hard vacuums. But they don't ! As a vacuum is softened, ALL of the photons arrive slower, not just the ones which staticically would encounter an atom, electron or something else along the way.

    It could be likened to a train traveling along some tracks.
    The tracks are always the same, yet the train does not travel at the same speed along the length of the track. The scenery effects how fast the train moves. Yet the train and the surrounds are unrelated. The train moves fast through the dessert, but not so fast through more highly populated areas.

    Why should the mere presence of matter effect the behaviour of passing light ? Does light effect the presence of 'other light' ?

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    Hi Trash,

    Thanx for this..part of the reason i joined...it sounded odd to me too..but I was *Told* this on another science forum and nobody disputed it...

    Never too late to learn!


    Thanx!


    Cheers, Kevin.

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    Sorry for the consecutive posts but I've been rereading you post trash and some stuff doesn't quite gel...

    When you say "there is a LOT of space between atoms" I presume you mean it is a vacuum? Hence wouldn't the wave and phase speed be the vacuum value?

    If we say that there is no absorption or reemission then presumably you are talking about some sort of diffraction effect around the atoms/molecules to account for the slowing (i.e. the photons travel further?)

    Ok...I'm now officially confused...what CAUSES the *slowing*?

    Cheers,

    Kevin.

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    and finally...




    Cheers,

    Kevin.

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    I think I know what they might have been talking about when they meant absorbtion and re-emission... but I can't think of the correct name for it. Something like electron pairing. It's also got a bit to do with zero point energy, the stuff of science fiction.
    Compton scattering is however something that does jump to mind on the subject.

    Even in a dense solid, there is a LOT of space between atoms. Why light passes through some materials better than others is not something I've stopped to consider since high school. The reason is probably very simple, but I cannot remember ever learning about it.

    I'm also a little puzzled as to why light should behave differently merely in the presence of matter and not even interact with it.

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