This is an A level AQA question:

A signal is to be transmitted along an optical fibre of length 1200m. The signal consists of square pulses of white light and this is to be transmitted along the centre of a fibre. Explain how the difference in refractive index results in a change in the pulse of white light by the time it leaves the fibre.

Blue ------------------- 1.467 -------------------------------- 425
Red -------------------- 1.459 ------------------------------- 660

What I am confused about is why would blue travel slower?

  • 1
    $\begingroup$ Hint: what's the definition of refractive index? $\endgroup$ – The Photon May 4 at 16:14
  • $\begingroup$ We know it does, in water, by the colors of the rainbow. $\endgroup$ – Whit3rd May 4 at 17:20

Take the wave model for light, we could imagine each oscillation to be like a person taking steps.

A "red" person takes longer strides than a "blue person". However they both walk at the same speed so long as they are walking on something like solid concrete (a vacuum for light).

When a red person and a blue person walk over a muddy bog, the red person makes it out first because the blue person must take more frequent steps; each of which slows him down.

As I stated at the start, I would be very interested to hear a more robust explanation of this phenomenon - this is just the way I could imagine it operating, but I may be wrong. I have read similar explanations in the past, but twisted them into this analogy which is the most clear for me. Again, I am not completely satisfied with this explanation though!


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