There are things you can’t see, and the ‘air’ is probably one of them. But, this demonstration of what is called the Schlieren effect can help you see air moving around any object. Or, in more scientific term, it can help you visualize changes in air density.
The Schlieren effect, also known as the Schlieren flow is based on the principle of refraction. This is how it works, as described by the creator of the video: The point source of light is aimed at the concave mirror and there, it gets reflected and focused onto the edge of a sharp object – which is mounted in front of the camera. Light refracted near the mirror is intercepted by the sharp-edged object, creating the illusion of a shadow that allows you to see air movement.