Experimenting the Speed of Light with Optical Fibers

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Discussion Overview

The discussion revolves around an experiment conducted to measure the speed of light using optical fibers of varying lengths. Participants explore the implications of the refractive index and total internal reflection on the measured speed, as well as the effects of fiber length on results.

Discussion Character

  • Experimental/applied
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Pascal reports experimental results showing that the speed of light measured through longer optical fibers is lower than expected, even after accounting for the refractive index of 1.49.
  • Some participants suggest that total internal reflection should be considered, as it may cause the light to travel a longer effective distance within the fiber.
  • One participant notes that the speed of individual photons remains constant at c, but interactions with the medium could slow down the overall propagation speed, especially over longer distances.
  • Another participant questions whether the refractive index adequately accounts for all interactions within the fiber, suggesting that impurities in longer fibers might contribute to slower speeds.
  • There is uncertainty regarding the relationship between fiber length, impurities, and the measured speed of light.

Areas of Agreement / Disagreement

Participants express differing views on the role of total internal reflection and the implications of the refractive index. The discussion remains unresolved, with multiple competing ideas about the factors affecting the speed of light in optical fibers.

Contextual Notes

Participants acknowledge potential limitations in their understanding of the physics involved, including the effects of impurities and the assumptions underlying the refractive index.

ATH500
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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:
10_m_essai_2.png

10_m_essai_3.png

10_m_deroulee.png

10_m_roulee.png


Here is the result for 20 meters:
20_m.png


For 50 meters:
50_m.png


For 0.60 meter:
delai_electronique_62_00ns_60c.png


For 0.615 meter:
delai_electronique_66_00ns_61_.png


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
 
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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.
 


So, does anyone else have any other ideas about this ?
 


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.
 


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...
 


ATH500 said:
Are you sure that's how it works ?

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.
 


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.
 

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