# Light Group Velocities

This was after enquiring about the delay of light propagation down a fibre. I come from a telecoms background so I wish to understand the delay for communications over a fibre.

Through back breaking arm chair reasoning (so much so I nodded off at one point) I surmised that light travels through a fibre slower than I originally thought (which was simply the speed of light through a given medium, which for fibre is roughly 0.64*c). I surmised that light is refracting all the way down the fibre so it has further to travel.

The above link was provided to me with an explanation that for multi mode fibre, the delay will be the group velocity of the light modes down the fibre. Once you get to this level of investigation the typical intuitive model for light breaks down.

I am such an immeasurably large physics novice though, I simply don't understand the contents of that link (no matter how much I wear a lab coat and ruffle up my hair, franticly scribbling the problem over and over on a black board. What are the odds of that!).

For single mode fibre I think it is quite simple (\$10 says I'm wrongly oversimplifying!) in that the light is travelling down the centre of a fibre as these tend to have a smaller core size (typically 9um). Multimode however is constantly refracting down the length of the fibre.

Can anyone simplify this into laymen's terms (which I expect is not feasible) or perhaps break it down into sections which I can then research individually?

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