View Full Version here: : Speed of light
20-06-2011, 11:18 AM
Does the speed of light change as it passes from one medium to another?
20-06-2011, 12:01 PM
Yes, I think the degree to which a medium slows light depends on the wavelength of the light, and maybe the refractive index of the medium?? Someone correct me if I am wrong.....:question:
20-06-2011, 12:10 PM
It's wavelength independent (a photon of red light travels at exactly the same speed as a photon of blue light). But you are correct in the refractive index (density) of the medium the light passes through. The denser the material, the slower it moves.
20-06-2011, 12:37 PM
There was an experiment at Harvard U where light was apparently slowed to 38 miles per hour. I think light was sent through a super cold cloud.
20-06-2011, 12:51 PM
Carl when light is refracted in a prism into different colors does this not suggest different speeds for different colors.?
20-06-2011, 01:06 PM
Yes, that is correct, if you're splitting "white" light up into its constituent colours and whilst that light is traveling through the prism. However, if you pass red light or blue light through a dense medium, the velocity at which the light particles enters the medium is always c, no matter the wavelength/frequency they're at. Once inside the medium though, they will behave differently according to the wavelength/frequency they possess. Once they leave the medium, the photons immediately resume their original velocity...c.
But that wasn't Ernie's question.
20-06-2011, 02:13 PM
An interesting perspective on the 'speed of light in a vacuum', (which is constant), and the speed of light in normal space, presents itself in the observation of pulsar light.
Ok, so pulsars, emit light pulses simultaneously over a wide range of frequencies (colours of light). But when these pulses arrive at Earth, the higher frequency components arrive before the lower frequencies. This 'dispersion' amongst the different received frequencies is theorised to be due to the ionised interstellar medium. (Space ain't a vacuum, eh ?)
So from the difference in arrival times, the number of electrons present along the path can be calculated (in electrons/square metre).
That's one way they can work out how (un)empty space is (in terms of normal matter). By looking at a whole heap of pulsars, they can combine the data to construct a model of how dense the interstellar matter is, along difference lines of sight.
Pretty cool, if you ask me.
(Just thought I'd share this fascinating snippet … )
20-06-2011, 02:26 PM
Hey, if they know about the speed of light, how fast is the speed of dark??:):P
20-06-2011, 02:38 PM
Thanks Carl I understood the original question and thanks for answering mine.
I don't know what tp think about the Harvard experiment. I read about it again today and it was 38 miles per hour as I remembered. I can't post a link but it is easy to find.But they sure have found a medium that slows light so it seems.
20-06-2011, 03:12 PM
That's easy to find out if you send a 1 us pulse of light down a great long light corridor. Is it still 1 us long when it reaches the end or has it been stretched or shrunk. Whatever follows the light must be the dark
22-06-2011, 06:08 PM
To slow the light, they had it pass through a cloud of sodium gas supercooled to 50 nanokelvins (very close to absolute zero) which laser light is shining constantly on it. 50 Nanokelvins made light go at 17m/s (meters/seconds)
22-06-2011, 06:41 PM
You could equally argue its the same as the speed of light since the dark is in front of the light, so it must travel at the same speed to get out of the way...
22-06-2011, 08:00 PM
It all depends upon how fast you open and close the fridge.
22-06-2011, 11:29 PM
Actually, that's not the right answer. If it's dark, you can't see, so how can you tell what speed it's moving at:):P
23-06-2011, 07:14 AM
Can't you smell how fast it goes?:P
Actually, there's no reason you can't casually jog along just behind the front of the light and be able to see the dark receeding.:D It works better than jogging backwards in front of the light front because you can't see the light front to know if you're in front of it, and next thing you know you collide with a light font coming up behind you and that can scare the willies out of you!:P
No, it's pure deduction... if the dark was slower, light would have to be slower... if dark was faster, there'd have to be something else that's not light and not dark in between the light and the dark...:P
Yeah, OK... but the speed of twilight is obviously slower at higher latitudes... this is where relativity comes in obviously...:P:P:P:P:P
Ah, now I see your point Carl! Dark is faster at higher latitudes!:thumbsup: We could be famous... Carl and Al's Theory of Relativity... no ones every come up with such an insight before!!!! :rofl:
Apologies Ernie. Ask a serious, valid question and the clowns come out to play... I hope you don't mind.:)
23-06-2011, 08:21 AM
Good onya, Al !
Good to see ya 'round these parts too !
23-06-2011, 11:48 AM
Have I opened a box? The answer seems to be refraction, so if the light bends around a star, then it is diverted because it passes through a denser medium adjacent to the star, so that is why when old navigators on Earth measure a low angle altitude, a correction is subtracted because the light is refracted in the denser medium of the Earth's atmosphere.
I dont mind the fun a bit Al!! I guess the dark bit bends as well***
23-06-2011, 11:56 AM
Light bends around a star because of the star's gravitational field. The environs around most stars are too dilute to cause any significant refraction of the light passing them. But you're correct in what you said about refraction in the lower levels of Earth's atmosphere.
24-06-2011, 06:40 PM
Thanks Carl. DO we have two answers? Refraction in a denser medium and/or gravitation. Does the gravitational field make the atmosphere around a star denser and cause refraction of light?
24-06-2011, 10:40 PM
the answer is it appears to bend around the star or anything else with a lot of Mass. Spacetime is curved by Massive objects.
24-06-2011, 10:51 PM
3rd potential answer....
25-06-2011, 02:05 AM
No, the gravitational field of the star bends the spacetime around the star. The light the travels through that warped spacetime follows the curvature of that spacetime.
Same as any of the planets or other objects that orbit the Sun travel a path in the curved space around the Sun. A straight line path, which in curved spacetime is a curved orbit:)
Very interesting debate.
25-06-2011, 10:36 AM
Ah! Space-Time is the reason. It sounds as if it was invented to explain the unexplainable!!! Another thought, if light is a particle then it has mass, so the gravitational field around the star attracts it. On the other hand if it is a wave, then it is refracted. Of course if we say that it is both, then the problem is solved; or is it? Wikipedia seems to have a bit each way.
25-06-2011, 10:59 AM
Photons have no mass. Whether they're a wave or a particle at the time you observer them, they still travel the curved path in the warped spacetime around a massive object. Remember, photons have a dual particle-wave nature.
27-06-2011, 04:59 PM
We have protons with no mass whether they be particles or waves, and they slide around warped space-time presumably at the speed of light. I am confused but that is due to my limited imagination. Thanks, I'll start another thread.
Photon. (http://en.wikipedia.org/wiki/Photon) (Wikipedia.)
Gravitational lens. (http://en.wikipedia.org/wiki/Gravitational_lens) (Wikipedia.)
Gravitational Lensing at KIPAC. (http://www.slac.stanford.edu/~pjm/lensing/)
Gravitational Lensing with Wineglasses. (http://www.slac.stanford.edu/~pjm/lensing/wineglasses/)
There are vast amounts of info about this subject available online. Noone with an Internet connection has any excuse for not being familiar with the basics. ;)
27-06-2011, 05:17 PM
Photons, not protons:)
vBulletin® v3.8.7, Copyright ©2000-2013, vBulletin Solutions, Inc.