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A piece of optical fibre is 150m long. Two rays of light travel along the fibre. One goes along the axis and the other is reflected off the boundary at the critical angle of 85 degrees. What is the difference in time taken by the two rays.

Speed of light = 3x10^8

Refractive index of core = 1.492

I was wondering if someone could possibly tell me how to find out the time taken by the ray of light that is reflected off the boundary cos i havent got a clue how to do it