Like the title says: when you see “heat rising” (the wavy/blurry area above a heat source), what are you actually seeing?
Bonus question: How does that cast a shadow? I noticed when I was opening my air fryer, I could see the shadow of the heat rising but, looking at the hot basket itself, couldn’t see it.
You’ve gotten some good answers explaining that heat changes the density, and therefore the index of refraction of air.
Fun fact: Schlieren Imaging allows one to photograph shockwaves by relying on the same effect. As a shockwave travels through air, it creates a region of high density, which can be imaged with this technique.