Answer:
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I halved the speed of light in vacuum to get the speed of light in the material, since the speed of light in vacuum is twice the speed of light in material.
The index of refraction is essentially the speed of light through a vacuum divided by the speed of light through given medium.
I attached some notes on refraction too, hopefully this helps :D
<span>One of the worst tsunamis to form occurred after an earthquake in Indonesia.
I hope this helps :)
Good Luck!</span>