eli5: when it’s hot why does it look like there’s water on the roads and why does it make a rippling effect.


eli5: when it’s hot why does it look like there’s water on the roads and why does it make a rippling effect.

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When the sun shines on the road, the road gets hot. Way hotter than the air above it. The road will then heat up the air directly above it, but the air slightly higher up will still be a bit cooler.

When there’s a sharp surface between hot and cold air, the light rays are bent at that surface. This is the same effect that happens on a surface between glass and air (which is how lenses work), or between air and water (which is why you can see your reflection in the water) or between hot and cold water (the ripples you see when you drop an ice cube into a glass of warm water). Directly above the road, this is strong enough to reflect the light rays; the “water” you see is actually the sky reflecting. A bit higher up, it just produces the rippling where hot and cooler air mix.

It’s the same thing that is called a mirage, or a fata morgana, when it occurs in the desert.

That’s a mirage and it’s because of 2 things.

1) evolution of your eye. Throughout evolution it’s been beneficial to assume that light rays coming from anywhere are always coming straight. Your brain naturally assumes that if you see an object, it’s exactly where you see it is, even if I tell you it’s an optical illusion. So any light coming into your eyes is assumed to come from a straight like where you’re looking.

2) the colder relatively and the road is hot. Hence there are different zones of temperature and density. This causes light from the sky to bend and refract in different ways. So, the blue sky light comes down and is bent towards your eyes due refraction.

So even though the light is coming from the air and nothing blue in front, your brain assumes that there must be something blue or glistening in front of you for you to be seeing light coming into your eyes. And the most common glistening and blue thing you assume? Water.

When light travels through objects of different densities, it gets bent/deflected, so to say. This is called refraction. [Here’s](https://static.sciencelearn.org.nz/images/images/000/000/049/full/Refraction-of-light-in-water20150805-30610-expmepedited.png?1625098472) an image of it. Light can also be reflected and/or absorbed, but we only need to focus on refraction for this.

Refraction can happen between pockets of air at different densities. Hot air is less dense than cold air (hence why hot air rises). Sunlight heats the road up (via radiation) and then the road heats the nearby air (via conduction). This new hot air rises and mixes with the colder air above it. Light refracts through these pockets and enters your eye. Your brain thinks that this light is coming from directly in front of you, but that light actually came from somewhere else and is just refracted toward you, and your brain processes it as this optical illusion that we call a mirage.