ELi5 How is there a reflection on the ground, that I see at a distance on a hot day?


On hot days while driving I see puddle-like reflections on the ground at a distance. Looked it up and saw things about pressure and light refractions… hoping to get a more understandable answer here.

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Let’s start with a more familiar example. If you’re trying to pick up an rock in a swimming pool or a fish tank, the rock isn’t actually where it appears. Light bends when it crosses between different mediums (glass, water, air) at an angle, because it moves at different speeds in glass vs. water vs. air. This means that different mediums can trick your eyes, which are used to light rays coming in straight from the object.

A mirage is just a different version of this, where you’re seeing an object at the wrong location because the light passed through different mediums. In this case, the air over the road is really hot, which makes it less dense than the cooler air higher up. Light rays from higher up get bent enough that they strike your eyes as if they were coming off the road. Rather than seeing the road, you see those objects higher up. This makes the road look like a reflective surface with an irregular border, which a reasonable brain calls “probably water.”