# Total internal reflection and refraction index in optical fiber and speed of light

1. Mar 19, 2009

### ATH500

I did an experiment on the speed of light with different length of optical fibers.
I know that the fiber has a refraction index of 1.49.
I would like to know if I need to take in account the total internal reflection of the optical fibers too.
Because when I use long fibers like for exemple 50 meters or 20 meter, it shows value
inferior to the speed of light even after having multiplied the refraction index of 1.49.
I get 218x10^6 m/s or 198 x 10^6....

But when I use optical fiber of short length, for example when I compare the difference between a 10 meters optical fiber and a 0.60 meter one, I got a result that is almost the same as the speed of light: 291326041 meters/second.

I will post the results I got when doing the experiment with the optical fibers and the oscilloscope.

Here are four results for a length of 10 meters:

Here is the result for 20 meters:

For 50 meters:

For 0.60 meter:

For 0.615 meter:

I would like to know why do I get value of the speed of light that are inferior the more the length of the fiber increase.

Thank you,
Pascal

2. Mar 19, 2009

### Oerg

Re: Total internal reflection and refraction index in optical fiber and speed of ligh

I think you do need to take into account of the total internal reflection since this means that the ray travels for a longer distance than the length of the fiber.

3. Apr 1, 2009

### ATH500

Re: Total internal reflection and refraction index in optical fiber and speed of ligh

4. Apr 1, 2009

### Danger

Re: Total internal reflection and refraction index in optical fiber and speed of ligh

Keep in mind that I'm no expert. The speed of individual photons, however, is always c. It's the interactions with atoms that slows down the propagation speed in a medium. It would make sense, then, that a longer path involves more interactions.

5. Apr 1, 2009

### ATH500

Re: Total internal reflection and refraction index in optical fiber and speed of ligh

Are you sure that's how it works ? I thought that the refraction index took care of all this and that it already take in account the interactions inside the same material. Because my optical fiber is made of the same material all along it must the refraction index should not change because every particle of that material already slows down 1.49 time the speed of light.

I wonder if the longer it gets the more impurities it has and if those impurities can slow it down...

6. Apr 2, 2009

### Danger

Re: Total internal reflection and refraction index in optical fiber and speed of ligh

No, I'm not; that's why I told you to keep in mind that I'm no expert. I was just giving you my best high-school drop-out's guess about the situation.

7. Apr 2, 2009

### ATH500

Re: Total internal reflection and refraction index in optical fiber and speed of ligh

It's a red LED input flashing at 100Hz. And when the receptor perceives light from the LED, it drops in voltage and we can see the effect on the oscilloscope. The LED correspond to the yellow graph and the receptor to the blue graph. I wonder if the impurities or the total internal reflection in the fiber has an effect.