# Light question

#### dusty8683

my uncle was describing something that showed light traveling at a slow speed and i had no idea how it worked. he said it was a clear solid plastic like tube wrapped around in a coil and when you shined a light in one end you could watch it enter the coil, spin around the coil, and then exit the other end. is that even possible? if so what was it and how did it work?

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#### anomaly

Yes, it is possible. Your uncle basically described the use and operation of fiber optics. Fiber optics are (basically) strands of glass optical fibers that are used to carry data signals (such as voice, video) in telecomm. networks. They also typically have a diameter of 60 &mu;m

Light can do some really interesting things, especially since it is able to bend around objects, reflect and refract off of (or because of) objects. So with these thoughts in mind, the idea that your uncle described can be done with light because of total internal reflection. Total internal reflection occurs when light attempts to move from a medium of with an higher index of refraction (n1) to a medium of lower index of refraction (n2). So, in order for the light to move through the glass tube, the light must undergo multiple internal reflection incidences. Basically, the light is bouncing off the internal walls of the glass as it is reflected - even around a curve. And to determine the likelihood of total internal reflection you will need to use Snell's law which is: sin &theta;c = n2/n1 ; where &theta;c is the critical angle (which is what you are looking for).

You wouldn't actually see it going through the coil, but would see it coming out the send. Even if you could see it in the coil, you still wouldn't be able to see the movement as you would with a clear hose with water and colored debris flowing through.

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