How does Schlieren photography work both generally and in viewing interactions between gasses of varying temperatures?

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How does Schlieren photography work both generally and in viewing interactions between gasses of varying temperatures?

In: Technology

Schlieren photography is an example of applied Fourier optics. A spatial filter is set up in a way which enhances phase changes of the light going through that filter. much like a Photoshop edge-enhancing effect, but in hardware.

The effect is, when photographed with such a setup, differences in optical phases are made visible. those may occur on (otherwise transparent) mediums of slightly different refractive indices.

Gases of different temperatures work well for this kind, as their temperature differs, their density
differs, and also their refractive index differs.