# Help with problem on dispersion

1. Feb 1, 2010

### lilmul123

1. The problem statement, all variables and given/known data

In many transparent materials, dispersion causes different colors (wavelengths) of light to travel at different speeds. This can cause problems in fiber-optic communications systems where pulses of light must travel very long distances in glass. Assuming a fiber is made of silicate crown glass (see figure below), calculate the difference in travel times that two short pulses of light take to travel 17.8 km in the fiber if the first pulse has a wavelength of 700 nm and the second pulse has a wavelength of 500 nm.

So far, I know that n at 500nm is 1.51, and n at 700nm is 1.5. I don't know what to do from here. I believe I have to use the speed of light somehow, but I don't know what I'm supposed to do with these n values. The only equation I really have is Snell's Law, but I'm not sure how these will help me solve the problem because I will only get some angle that won't really tell me anything about a length or anything. Can anyone give me a nudge?

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