Schlieren imaging

I’ve been sent this link to a movie of a shock wave from a trombone.  You’ve got to feel sorry for the poor clarinetist who is sitting in front.

This sort of thing can be done neatly with the method of schlieren imagaing. (See some more examples here) This is a ‘simple’ way of picking up changes in refractive index of something (usually air or water) that is transparent, in a way that a normal photo would not. It’s done by putting a knife-edge at the back focal plane of the lens, which cuts out some light rays in such a way that the end effect is to give contrast to the places where refractive index changes rapidly. These places are often shock waves, where the density of air is large. A proper mathematical description gets a bit horrendous (, which involves the wonderful mathematical beast the Hilbert Transform), so I’ll leave the discussion there.  But if you don’t mind taking your camera apart and reconstructing it, in principle at least it’s not a difficult thing to do.

P.S. I’ve been rushing through my tax return trying to get it done before departing overseas at the end of this week  (…the blog may take a back seat until the end of June…) and discover that the IRD now refer to ‘natural persons’.  What other sort of person is there?

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