Nothing but air
With a dark field illumination (schlieren optics), the smallest changes in air density (refractive index) are made visible. The apparatus is so large that the heat radiation of a human head becomes visible. With a concave mirror, the light from a point light source is made parallel (focusing a light source on a slit) and imaged on a disc with a second concave mirror with a large focal length. The object to be imaged lies between the two concave mirrors and is imaged on a screen by the second concave mirror.

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