# Measuring the speed of light

by ravisastry
Tags: light, measuring, speed
 P: 52 Hi, how is the speed of light measured in objects such as water, glass, diamond etc ? what are the tools and techniques used. thanks in advance !
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 Quote by ravisastry Hi, how is the speed of light measured in objects such as water, glass, diamond etc ? what are the tools and techniques used. thanks in advance !
Welcome to Physics Forums.

Just out of interest, why do you want to know? Why are you interested in the speed of light in transparent materials?
 P: 52 just wanted to know how speed of light is calculated in places other than vaccumm...who knows, the answer i get, might help in formulating some new theory !
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## Measuring the speed of light

In terms of materials a viable way would be interferometry
 P: 52 thanks...now, ill google "interferometry"
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 Quote by ravisastry just wanted to know how speed of light is calculated in places other than vaccumm...who knows, the answer i get, might help in formulating some new theory !
I look forward to it.
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 Quote by ravisastry Hi, how is the speed of light measured in objects such as water, glass, diamond etc ? what are the tools and techniques used. thanks in advance !
If you don't mind an indirect method, you can use Snell's law and measurent of refraction angles with oblique incidence. This should work well with the highly transparent materials you mentioned.
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 Quote by Feldoh In terms of materials a viable way would be interferometry
That's good for gases and fluids. Not so much for solid materials.

If you have a thick enough slab of material, and you send the beam through it a few times with mirrors, you can just time it directly.

One of the experiments done in the Intermediate Physics Lab in our department is measurement of speed of light using an oscilloscope, and LED, a photo-diode, and some mirrors. You basically use the oscilloscope to measure time it takes a pulse of light to travel a certain distance. You can get something like 10-3 precision with that.
 P: 52 guys, im not getting what i was looking for "http://en.wikipedia.org/wiki/Interferometry" the link doesnt tell about speed of light measurement. k^2 ... what i was looking for is, say you have a glass slab of 1 cm thickness. how much time would like to traverse this and roughly, how many atoms would it encounter. we can take any substance, not just glass. i'd like to know the time gap between a photon absorption and re-emission in any atom. do we have an experiment or ready made answer for my question :)
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 Quote by K^2 That's good for gases and fluids. Not so much for solid materials.
You might have to get into atomic physics or maybe even quantum mechanics in order to get your answer. I am over my head just by posting the foregoing, so I can be of no further help. Believe, though, that any reflection/refraction is a consequence of electron shell structure.
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 Quote by K^2 That's good for gases and fluids. Not so much for solid materials
For solids you can use the Brewster angle. Kind of fun to measure the speed of light in coal.
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 Quote by Vanadium 50 Kind of fun to measure the speed of light in coal.
That obviously brings to mind that there might be a new method of medical stool analysis to be patented here. Damn!, but I hate you!

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