Page 3

The "refractive index" of a medium tells us how much its slows down the waves that enter it.

It is equal to the ratio of the speed before it enters to the speed inside.

so if light travels at a speed v inside a certain type of glass then its refractive index, n = c / v

The refractive index is always a number bigger than 1. It has to be as v must always be less than c.

The refractive index is also equal to the ratio of sin i to sin r. This is called Snell's law

For your entertainment, and to show you how easy it is, I have trained my pet ant, Kevin, to perform a calculation for you.

Light enters a type of plastic at an angle of 45⁰, inside the plastic it travels at 32⁰. Calculate the refractive index of the plastic.

It's a little bit trickier when we know the refractive index and we want to calculate i or r. (even Kevin struggles with them) But you'll get used to it with a bit of practise.

Why do we need to know all this stuff? Lets go to Page 4 to find out.