Is light faster in vacuum than it does in water?

AI Thread Summary
Light travels faster in a vacuum than in water, with a speed of approximately 300,000 km/s in a vacuum compared to about 225,000 km/s in water due to interactions with atoms and molecules. This slower speed in water is explained by the absorption and re-emission of photons, which creates delays in the light's passage. Snell's Law can be used to measure the speed of light in different media by calculating the refractive index, which is approximately 1.33 for water. While light's speed appears to decrease in media, the fundamental speed of light remains constant at "c" in a vacuum. Understanding these principles is crucial for grasping concepts in optics and physics.
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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
 
Science news on Phys.org
Snell's Law
http://www.ms.uky.edu/~skim/SpeedOfLight/
 
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heaven eye said:
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
No. Light is always slower in water.

Pete
 
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heaven eye said:
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).

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 --->
http://www.rpi.edu/dept/phys/Dept2/APPhys1/optics/optics/node4.html

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

You can also determine the speed of light c through a medium, by measuring the permittivity \epsilon and permeability \mu of the medium; and using the relationship c = \frac {1}{\sqrt {\mu \epsilon}} But then that is another story :wink:
 
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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
 
qutache thank you very much
 
am sorry but another question

why don't scientists study light in solid be cause its much slower and know some of its describes to help them and help in relativity theory
 
heaven eye said:
am sorry but another question

why don't scientists study light in solid be cause its much slower and know some of its describes to help them and help in relativity theory
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 http://www.sno.phy.queensu.ca/sno/sno2.html#interactions)

[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.]

AM
 
Ouabache said:
As pmb_phy mentioned and I agree, light is slower through water..

That is incorrect. Light always has speed v equal to c


However through any other medium, light encounters atoms and molecules of that medium and becomes scattered,

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

resulting in a slower speed.

Untrue, read above

marlon
 
  • #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!
 
  • #11
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...

Daniel.
 
  • #12
Maybe I am nto understanding you marlon but, your saying the speed of light is ALWAYS c? c being 3x10^8?
 
  • #13
whozum said:
Maybe I am nto understanding you marlon but, your saying the speed of light is ALWAYS c? c being 3x10^8?
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.
 
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  • #14
Integral said:
Yes, he is.
The speed of light appears to be slower in media because it is not continually propagating. Sort of like like driving done 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.

Indeed,

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

thanks...

regards
marlon
 
  • #15
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.
 
  • #16
That's why we invented quantum theories.To explain why light behaves like it does is one of the reasons...

Daniel.
 
  • #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 can't be passed?
 
  • #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 can't . therefore there is diffrent betweent light and photon

>>>>>is that right!
 
  • #19
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.

Daniel.
 
  • #20
so what about shell's theory !

thank you mr. dextercioby

and i want to tell you that iam so beginner in physics that's why i asked (silly questions :blushing: ) be cause i don't know but funamental in physics
 
  • #21
That's Snellius (Latinized name).He rediscovered the refraction law and wrote it in terms of "sine" function (unavailable to Claudius Ptolemeu).

Learn optics first.

Daniel.
 
  • #22
How does it come that we can see an object so clear through water then?
The emision can usually occur in any direction but this explanation seems to build on the fact that the emision occurs in the same (or nearly the same) direction as the absorbtion occurred in, which contradicts my, and others, views of optics.
 
  • #23
Integral said:
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.

I don't think that light can stop since it's speed is fixed and it can't be slowed down. Speed of light is lesser in medium other than vaccum, because light is absorbed and re-emitted in the medium repeatedly. This absorbtion and emission doesn't occur instantly so results in a delay, that's why ...:biggrin:
 
  • #24
Light always propagates at c, it does not travel slower in a medium. Which is explained by the FAQ's funnily enough, this question is like does .99999999999999999999999.=1. In number theory.

https://www.physicsforums.com/showpost.php?p=899393&postcount=4
 
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  • #25
Hello to all.

Does light absolutely need space for it to move/propagate ? When I say space I mean a vacuum (I think).

If we remove all the matter from this universe, can the light move through what remained ?
 
  • #26
Light dies not need a medium to propagate in. But even if you remove all the matter from say an area, space does not remain empty, particles pop in and out of existence anyway, consequently no area of space is a perfect vacuum. However let's assume it was, or that you could remove all the matter from the universe, in that case it would be able to move through the void.
 
  • #27
Hamble said:
The emision can usually occur in any direction but this explanation seems to build on the fact that the emision occurs in the same (or nearly the same) direction as the absorbtion occurred in, which contradicts my, and others, views of optics.

Transmission of light (photons) through a transparent medium is actually a rather complicated process. You can't think of it simply in terms of scattering off of individual atoms or molecules. The atoms/molecules interact collectively instead of individually. To get an idea of what's happening in a solid, at least, see post #4 in the Physics Forums FAQ which is stickied to the top of this forum.
 
  • #28
Schrodinger's Dog said:
Light dies not need a medium to propagate in. But even if you remove all the matter from say an area, space does not remain empty, particles pop in and out of existence anyway, consequently no area of space is a perfect vacuum. However let's assume it was, or that you could remove all the matter from the universe, in that case it would be able to move through the void.
Ok. But isn't void actually the empty space itself ? If I assume that space in this universe is finite, and the beam of light reaches the end of space so to say, were will it go? (or in fact it goes in circles?) Can light escape outside of space so to say?
 
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  • #29
Nick666 said:
Ok. But isn't void actually the empty space itself ? If I assume that space in this universe is finite, and the beam of light reaches the end of space so to say, were will it go? (or in fact it goes in circles?) Can light escape outside of space so to say?

The universe is general understood to be infinite, there is nothing outside of the universe/space. Think of space as the surface of an ever expanding basketball.
 
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  • #30
I've only got high school physics at the moment, so bare with me.


I'm confused? If you timed 2 beams of light traveling the same distance, yet one was in a vacuum, the other say something very dense, like a diamond. Which would reach the end first? The one in the denser medium is slower than in the vacuum yes? Or does time slow down somewhere :D ??
 
  • #31
Correct me if I am wrong but light always travels at the same speed, it just takes longer to travel through a body of water because it is not traveling along a straight path as it would in a vac...
 
  • #32
Joza said:
I've only got high school physics at the moment, so bare with me.


I'm confused? If you timed 2 beams of light traveling the same distance, yet one was in a vacuum, the other say something very dense, like a diamond. Which would reach the end first? The one in the denser medium is slower than in the vacuum yes? Or does time slow down somewhere :D ??

the beam through the denser medium arrives later. Using a laser beam this is effect observed as an interference pattern which occurs if the beams are recombined. The term to research would be OPTICAL PATH LENGTH.
 
  • #33
Does that mean that the speed of light is NOT a constant?

Yet as I understand it is, always being 3 by 10^8 ms^-1.

Anyone care to explain this?
 
  • #34
Please read the FAQ - https://www.physicsforums.com/showpost.php?p=899393&postcount=4
 
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  • #35
I am not happy with this ancient thread being resurected, then getting replys by people who obviouly have not read it.

LOCKED !
 

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