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I'm really stuck on the following question. I think that I have got part a alright, but I've no idea what to do for part b. Any help would be really appreciated.

**The refractive index of glass is 1.643 for blue light, and 1.618 for red.**

(a) Calculate the difference in the speed of light in the glass for the two colours.

(a) Calculate the difference in the speed of light in the glass for the two colours.

Okay, for this one I got that if n = C/V, for blue light V = 1.83 x 10^8 and for red light V = 1.85 x 10 ^8. Therefore, the difference is 2 x 10^6 m.s^1.

**(b) If a very short pulse of white light is sent in a one end of an optical fibre made of this glass of length 1 km, find the time interval between the arrival at the other end of the red and blue light.**

Thank you.

Cathy