Refraction is the bending of light.
Here is a simple experiment to show refraction.
Place a penny (or other flat object) under a glass. Stand so that you can see the penny through the side of the cup and poor water in. What happened to the penny?
In the beginning of the experiment the light from the penny is traveling directly to your eyes (technically it bends a little as it goes through the glass, but we will ignore that for now).
When you pour the water in, it causes the light to refract and it does not exist through the side of the glass.
So why does light refract?
Light travels in a wave, so it has a thickness. As one side of the light wave hits a substance with a different density, that side slows down. That causes the light wave to turn.
Imagine an axle with 2 wheels rolling on a smooth surface. If one of the wheels hit sand, it will slow down and cause the axle to turn toward the sand. This is the same way that light refracts – it will turn toward the denser substance.
Light can also refract when it passes through air with different densities.
A couple years ago, we did an experiment where we made big fireballs. (I guess I should write a post about that one too).
In the first picture, you see a fireball. The second picture was taken just after the fire went out. The air was still hot from the fire and therefore had a lower density. This caused the light to refract and make the wavy appearance.
This is the same thing that happens when you look at a road on a hot summer day and see wavy lines above it. The road is heating the air and causing the light to refract. Refraction also causes mirages in the desert as the hot sand warms the air.
I hope you have fun experimenting with refraction.
Check out the rest of the properties of light series here.