The film can move so fast that itt's speed exceeds the wave speed of the film.
The width of the channel is about 2 cm, and the shock waves are induced by inserting the tip of a needle through the film. The needle is not quite visible. These photos are tolerable, but once I build a better illumination system here they will be more uniform in contrast. I estimate the film speed at 3 to 4 m/s and the film thickness at 5 to 6 micrometers.
From left to right we have a slightly subsonic flow, where the film "piles up" in front of the needle and buckles out of the plane of the film. Then we have a slightly supersonic flow, and finally a more supersonic flow. The shock waves are undulations of the film perpendicular to the plane of the film, and hence. They are visible because they don't reflect light back into the camera. The other fringes you see are interference fringes due to thickness changes in the film. These are visible mainly because I used monochromatic light (Sodium lamp) to illuminate the film.
For this second picture I did some image processing to get rid of the non uniform illumination. It enhances some features that are difficult to see in the unprocessed photo.
