When air is heated by a fire or a hot surface, swirls of hot air rise up through cooler air above. As the hot and cool air mix, light that’s traveling through the air goes in and out of many swirls and pockets of hot and cool air.
Light takes a slightly different path through hot air than it does through cool air. So the light is bent back and forth as it goes through mixing air. As you look through this mixing air at something beyond it, the light that comes from the object to your eye is bent back and forth—which makes the object look wavy or blurry.
Photo by: iStock|petrograd99