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.
In a not super scientific explanation, hot air rises and is less dense than cold air, this changes how light passes through it compared to the rest of the air. Shadows are essentially less light dense areas, so by changing how light passes through it you naturally get less dense regions behind the distortion making it darker than the other areas, aka a shadow. Air, being transparent though, there is less distortion than an opaque object, such as yourself, which has greater ability to distort (or rather, prevent) light in that area. (Your shadow)