Illusion enthusiast Brusspup, who’s known for his amazing videos, performed a fascinating experiment in which changes in air density were shown using a concave mirror, a light source, a razor blade and a camera. This is known as the Schlieren flow visualization.
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.