# How to find the speed of light practically?

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1. Nov 29, 2016

### Andrii Iermolaiev

Currently, i'm doing the CREST award for physics, and my topic is about different methods of finding out the refractive index of a medium. I have already tested out few ways, and now i am thinking about a method where a formula n=c/v will be used(n-refractive index, c-speed of light in vacuum, v-speed of light in medium). So, what i am looking for is the way to carry out an experiment where i will find the speed of light in medium. I have thought about using light-sensitive screens, but i suppose that difference will be so insignificant that results will be far away from being accurate. If anyone has any ideas i would be happy to hear them.

2. Nov 29, 2016

### Orodruin

Staff Emeritus
You could probably set up some time of flight measurement for a light pulse. Compare the result with and without medium.

3. Nov 29, 2016

### Staff: Mentor

You might be able to use Foucault's method. When I was an undergraduate, we used it to measure the speed of light in air, but didn't try it with a tank of water also. Instead of a light source with a slit, we used a laser beam.

4. Nov 30, 2016

### sophiecentaur

c is a very high value and you need to use either a long distance or be able to measure a short time interval (stating the obvious). Electronic methods can cope with this without too much trouble. To get a long distance, you can use two mirrors and have the light follow a zig zag path. Ten reflections with mirrors spaced at 5m will give a path length of 100m. The time delay in air / vacuum will be about 30μs, which can be measured easily with simple lab equipment. Use a pulsed laser and photo diodes, one at the and one at the end. You can display the pulses that the diodes produce on an oscilloscope with two traces. The pulses may be 'rounded off' due to the limited bandwidth of the system but you can still measure the time between the rising edges.
The same thing could be achieved with the mirrors under water (school swimming pool, for instance?). The electronics, light source and diodes could be above the water and mirrors used to introduce the laser beam into the measurement range.

5. Nov 30, 2016

Staff Emeritus
I would instead try and measure v - c. Pulse an LED, direct some light through the medium and some through air, and have both pulses impinge on your sensor. The time difference in the pulses gives you v-c. If you have a ten foot path, you need nanosecond-level sampling. This used to be expensive, but now you can buy a scope that does this at Fry's.

6. Jul 10, 2017

### Andrii Iermolaiev

THANK YOU VERY MUCH!!!

7. Jul 10, 2017

### rumborak

That's a really ingenious method, I love it.

8. Jul 11, 2017

### sophiecentaur

Certainly worth a try if you have the resources to by stuff but even small costs could be too much in (UK) education. There is always a trade off between path length, which will reduce the signal and timing resolution. For simple electronic light modulation detection and scope display, you are limited to not much less than microsecond accuracies and the implies the need for paths of several hundred metres. Lasers make the problem of long distance much less and a 'there and back' measurement of 200m would be easy to achieve across an open field or between two buildings. Multiple reflections would only be necessary if you are short of real estate. Lining up the laser and mirror would not be a huge problem.