This video is damn cool, and combines the ever popular high-speed camera, with so-called Schlieren pictures.

The basic idea here is quite intuitive and in fact you’ve probably seen these effects many times in your life. What’s happening is that although we can not see shock waves directly, they create different densities in in the air, which then in turn has a different index of refraction (which means that light that hits low density air is bent in a different way from light that hits the high density). The result is that you get a sort of shadow-picture, that then shows how the air is reacting. An example of this that you’ve probably seen, is the shadow of hot air rising from a candle. If you place a light behind the candle, and look at the shadow hitting the table, you’ll see how hot air is moving around the candle, even though it is naked to the visible eye invisible to the naked eye. This is more or less what they are doing in this video, although they obviously have a very refined technique to get great videos like these.