You can see the effect of refraction with a easy experiment.
Place a penny in an opaque container so that the penny is not visible at that point, but you can see the top. Keeping your head in the same point, slowly fill the container up with water, and soon you should be able to see the coin.
Important formulae Edit
The important refraction formulae include:
speed of light in a vacuum / speed of light in the material = n
Bearing in mind that n (refractive index) has no units and the speed of light in a vacuum (c) is 3.0 x 10^8.