# Path difference in Fibre Optics

#### Graecia

Hi,
This is a question from a homework which has stumped me:
Light is travelling along a fibre optic cable of refractive index 1.52, the refractive index of the cladding is 1.51 and the critical angle between the two is 82.
a) what is the most direct route for light along the fibre-optic cable
b)The longest route is for light rays travelling at the critical angle. Calculate the path difference between the two rays after travelliing 10km of the cable

I guessed that the most direct route would be straight down the fibre optic cable, and for b i calculated the speed of light in the core to be 1.97x10^8 m/s

Any help would be greatly appreciated,

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