substance/material	refraction index at 20°C.
air	1
water	1,33
glass	approx. 1,5
diamond	2,47

A Model Of Refraction

This model helps to predict which direction a light ray takes when it meets the interface of two materials and is refracted there.
We need your imagination for this model. Imagine you pull a car! You begin pulling on a street made of asphalt, then at the side of the street you have to cross a strip of sand with your cart and finally you reach the grass of a meadow onto which you pull your cart, too. However you do not walk and pull your cart straight ahead, you actually take an angular path towards the sand and the grass. This means that the left wheel of the cart will roll onto the sand first.

It's much easier to pull the cart across the asphalt than it is on sand. This means the wheel that meets the sand first, the left wheel in our case, is slowed down quite much. The other wheel continues to move at its old speed. This causes the right side of the cart to "overtake" the left side of the cart - the cart turns and changes its direction.
On grass the wheels roll easier than on sand. Here the left wheel rolls faster than the right wheel which makes the cart turn and change its direction more to the right.

This is very similar to what happens to light at interfaces of two materials. Surfaces that are hard to drive on resemble materials with a high refraction index - it's also harder for light to move through such materials or substance. The bigger the difference between the refraction indexes of the two substances, the stronger the refraction. The cart's direction after the bend depends on what surface follows onto the old one - a good surfaced followed by a bad surface or the other way around. With light it's quite similar. The new course the light rays take after the refraction is also determined on the order of substances.