ishalon

When light travels through air... wouldn't the air particles obstruct it, or at least a lot of it?

[yes, i should try to read something before i ask questions]

Coragyps

The speed of light is slower in something than it is in nothing: in any medium, light (or any electromagnetic radiation) travels at (the speed of light in a vacuum) divided by (the refractive index of the medium.) The refractive index for water is 1.33, for window glass about 1.50, for air maybe 1.001 or so. And yes, air can be said to obstruct light, both by absorption of some wavelengths by the molecules themselves and by scattering of the shorter wavelengths. The absorption is (mostly) in the parts of the EM spectrum our eyes can't see, like the short-wave ultraviolet. The scattering is why we get red sunsets and blue skies - the blue gets scattered while the red comes on through.

Starboy

Thank you Coragyps for that explanation, I would also like to add that from the point of view of General Relativity, the presence of matter bends space/time. This creates what we call gravity but as Einstein so cleverly deduced with one of his famous thought experiments is just another form of acceleration. Light as with all matter that travels in bent space/time would be accelerating.

Adios

Starboy

[ June 29, 2002: Message edited by: gkochanowsky ]</p>

Michael Ledo

In the nuclear industry ther is something known as the Cerenkov (spelling?) effect. Spent nuclear fuel will give off an eerie dark blue glow. I call it "neutron blue". We tell folks that it is the color of the neutrons leaking out of the core. In reality it is caused by particles in water going FASTER than the speed of light in water. When the Soviet Union had its mishap some people claimed they could see fuel give off a green glow in the river. Many scientist scoffed or dismissed this claim. However, given the murky conditions of the water and having seen fuel glow, I would not doubt their stories.

[ June 29, 2002: Message edited by: Michael Ledo ]</p>

Friar Bellows

<a href="http://www.nobel.se/physics/laureates/1958/press.html" target="_blank">Cerenkov Effect</a>

The "C" is pronounced "Ch".

So yes indeed boys and girls, particles can go faster than the speed of light. But not in a vacuum. The limiting speed from relativity is the speed of a light in a vacuum.

Straight Hate

So that would explain the discovery (of several months ago) that light travels 'faster' through cesium vapour, would it not?

Mike H

I thought the experimenters who had gotten a light pulse to go "faster than light" through a cesium vapor had reconciled it with the special theory of relativity by determining it was the group velocity propagating faster than c, not the phase velocity. And as I understand that, information can only be transmitted when considering the phase velocity, as the group velocity (basically the velocity of the wavepacket as a whole)loses all extractable information regarding amplitude and frequency, which means causality is preserved. I know Griffiths mentions the difference between phase and group velocity in his undergrad EM book, and there's a discussion of it in Jackson (the horror! ) somewhere.

I could very well be wrong though.

Boro Nut

I'm not sure, but my experiments suggest it decelerates when the temperature approaches zero. I've experimented since a boy to trap light in a shoe box but find you cannot get the lid on quick enough. However, it seems to work no bother with a fridge.

Boro Nut

Friar Bellows

Here's a good web page (with java applet) to learn about group velocity:

<a href="http://www.netspace.net.au/~gregegan/APPLETS/20/20.html" target="_blank">"Subluminal" Java Applet</a>

Mageth

Light has also been slowed to <a href="http://www.news.harvard.edu/gazette/1999/02.18/light.html" target="_blank">38 miles an hour.</a>