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[Phys-L] Travel distance in a waveguide.



We have an apparatus for measuring the speed of light in which a red LED
is pulsed at about 800 kHz; the pulses travel through a 20-meter fiber
optic cable; an oscilloscope is used to measure the time the light
pulses take to go through the cable.

The company advertizes the refractive index for the fiber is 1.50 for
the LED being used. The directions for the students say to calculate
the observed velocity as delta-x over delta-t using the length of the
cable for delta-x. Then find c = nv using 1.50 for n.

The students originally think a fiber optic cable works by multiple
reflections, and therefore suspect the actual pathlength of the light is
much greater than 20 meters. I have told them that fiber optic cables
work more like a wave guide; however, I realized I don't know exactly
what that means in terms of path length because I have no experience
working with wave guides.

My first question for this forum is... when a wave goes through a
waveguide, what is the effective length of the guide. Is it just the
physical length?

My second question is... it is my understanding that an optical fiber is
constructed so the refractive index is continuously graded from a higher
refractive index at the center to a lower refractive index near the
outside. If this is true, what does it mean to refer to a cable as
having a particular refractive index?

Putting my first two questions together, my third question becomes... is
this apparatus a fraud? Has the company simply said the refractive
index of the fiber is 1.50 so the calculation for c comes out correct?
That is, might we say n = 1.50 is an effective refractive index that we
determine from knowing c, rather than vice-versa?


Michael D. Edmiston, Ph.D.
Professor of Physics and Chemistry
Bluffton University
Bluffton, OH 45817
(419)-358-3270
edmiston@bluffton.edu