This is a demonstration of the Schlieren effect. This setup allows you to see changes in air density. The point light source is aimed at the concave mirror. The concave mirror reflects to a focal point. There you use a sharp edged object to partially block the light which helps create a shadow effect that allows you to see air movement.
Big Thank you to: Ruud Verdaasdonk Ko van der Lubbe Phil Koken VUMC