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Old Feb 27th 2009, 03:11 PM   #1
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Speed of light

Hi

From my understanding light can only travel at the speed of light. It is impossible for it to travel any slower than this. I know in optics it appears that light travels slower than this speed, in glass for example. SO my question is how is this possible? From my understading this appears to be the case because the particles in the glass absorbs and then remit the light, thus appearing to slow down light, as this does not happen instantaneously. Are my assumptions correct and if so could you please explain what is happening. If they are not correct, could you also explain why this is not the case. I look forward to hearing your replies.
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Old Mar 1st 2009, 12:27 PM   #2
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Im not exactly sure of the specific physics behind it but you may be interested in one of these articles.

Harvard Gazette: Researchers now able to stop, restart light

Putting the Brakes on Light Speed - washingtonpost.com
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Old Mar 2nd 2009, 11:35 AM   #3
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I am aware that light appears to be slowed down when passing through certain mediums but as far as I am aware it is still travelling at the speed of light but it appears slower de to the fact that it is being absorbed and remitted by the atoms in the medium. I was hoping that someone could verify this or explain why this is not the case.

On a different note, we have actually managed to trap light. This is done using Bose Einstein condensates which have temperatures billionths of a degree warmer than absolute zero. I know they are trying to use them to model black holes.
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Old Jul 18th 2010, 01:13 AM   #4
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Why is glass transparent?

Light can pass through glass because it is transparent. Why is glass transparent? Is it has something to do with light? Light cannot pass through steel, for example.
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Old Aug 1st 2010, 06:49 AM   #5
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It has to do with the structure of the crystals that make up glass and steel. The glass crystals are more "open" and so allow more light through (although they absorb some).

This is connected, by the way, with the fact that glass crystals are not very strong and deform easily. That is why glass is, in a sense, a "quasi- liquid". Glass window panes that have been standing vertically for a very long time will be thicker at the bottom than at the top- they have "run" downward.
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Old Aug 1st 2010, 07:08 PM   #6
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Originally Posted by cas34 View Post
I am aware that light appears to be slowed down when passing through certain mediums but as far as I am aware it is still travelling at the speed of light but it appears slower de to the fact that it is being absorbed and remitted by the atoms in the medium. I was hoping that someone could verify this or explain why this is not the case.

On a different note, we have actually managed to trap light. This is done using Bose Einstein condensates which have temperatures billionths of a degree warmer than absolute zero. I know they are trying to use them to model black holes.
You are right. Light takes longer time to travel a certain distance inside the glass compared to that in air due to absorption and re emission. So it appears to be moving slower. In between the atoms the space is empty. In that space the light travels with the speed of light.
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Old Aug 26th 2010, 01:53 PM   #7
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One way of thinking about it that a teacher of mine use is to think of a man and a woman walking down a city street lined with clothing shops. The man is light traveling in vacuum and the woman is light traveling through a medium, for example glass. In a sense, the motion is pretty much the same. No, I will not say sorry for chauvinism.
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Old Sep 6th 2010, 10:32 PM   #8
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Speed Of Light

Consider the flow of water.

A water is flowing in a pipe with some speed but very fast. But if you put a small stone inside a pipe it will slow down the speed of water. This is due the medium that you put in the travelling way of water.

Same in the case of light. A speed of light that is ideally taken is 3x10^8 (so far as I remember). But this is the ideal situation i.e in the vacuum surface. In air there are molecules and dust particles which slow down the speed of light.

Same happens when light travels through glass, the thicker the glass, the more slow down of speed of light.

Last edited by arbolis; Sep 7th 2010 at 07:55 AM. Reason: incomplete
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Old Oct 7th 2010, 04:33 AM   #9
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According to Einstein's Theory of Special Relativity, the speed of light is a constant (c): light will always travel at 3.0*10^8 m/s no matter how fast you are traveling. So when you turn on the headlights, you will still see the light beams traveling away from you at the speed of light. However, driving at light speed is impossible, because traveling at that velocity requires an infinite amount of energy. And unfortunately, the fuel to provide infinite energy wouldn't fit in the entire universe, much less your Satellite's fuel tank.
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Old Jan 12th 2011, 02:42 AM   #10
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Hi

From my understanding
the speed of light = 299 792 458 m / s

Last edited by pethichammer; Jan 17th 2011 at 02:54 AM.
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