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Old 04-21-2004, 06:49 PM   #1
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Fiber Optic Cables

Many companies use fibre optic cables in their server farms and use light a instead of electricity to send packets. What I want to know is whether the frequency as it travels down the cable is at light speed or if it was somebody bluffing me a year ago?
Old 04-21-2004, 08:15 PM   #2
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Light always travels at the speed of light.
However the speed of light varies depending on the medium. (glass vs. air vs. vacuum, etc.)

While light has a frequency of its own, with regard to information you typically are talking about a carrier frequency and a baseband frequency. (Simple analogy, on a dark night if i flash you a message in morse code with a flashlight, the light is travelling at the speed of light, while the morse code (information) is travelling much slower)

Symbols(groups of bits) travel at the baseband (information) frequency, which is less than the carrier frequnecy, which is typically much less than the speed of light.

So yes, the light in the fiber optic cable travels at the speed of light (in glass). However, the information it carries does not.

Fiber optics gets is boost in speed over copper wire because there is less thermal noise in the wire (notice how metals heat up when carrying current). Less noise means symbols can be sent more rapidly and still be reliably decoded. (With the flashlight example, if it is dusk instead of night, you will have more trouble reading the flashes of light, so i will have to signal you at a slower rate in order for you to be able to figure out what I am sending).

If someone has a better grasp of the physics here, please correct me.
Old 04-21-2004, 09:05 PM   #3
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Electricity travels really close to the speed of light. Secondly yeah light travels at the speed of light in fiberoptics(i believe faster than the speed in a vaccum).
Old 04-21-2004, 09:15 PM   #4
Brane Ded
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Originally posted by Stack
Secondly yeah light travels at the speed of light in fiberoptics(i believe faster than the speed in a vaccum).
Hmmm, don't know about that.

The speed of light depends on the material that the light moves through - for example: light moves slower in water, glass and through the atmosphere than in a vacuum.
Old 04-22-2004, 03:03 AM   #5
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This is becoming a philosophical question.....

If light moves slower through dense materials, can it still be travelling at the speed of light? Discuss.

Old 04-22-2004, 04:04 AM   #6
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I think the answer is that light can only ever travel at the speed it is travelling so light is always going at the speed of light. To my limited knowledge of physics I thought it was impossible to slow light down and I am fairly confident in this. You can bend light and reflect and refract it but I very sure you cannot slow it down. However if you could slow it down it would be still travelling at the speed of light so it is a mute point.
Old 04-22-2004, 05:01 AM   #7
Brane Ded
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You can slow it down. Not by any amount you would notice, but you can.


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