People sometimes label a mirage as an illusion. But, in fact, a mirage is not an illusion. Your mind creates an illusion. A mirage can be explained by the physics of Earth’s atmosphere.
You know how a spoon in a glass of water can appear bent. Our atmosphere can cause some distant images to undergo a similar effect called ‘refraction.’ Close to the ground, refraction is strongly affected by variations in temperature. If the temperature goes up as you get higher above the ground, you might see what’s known as a ‘superior mirage.’ That means an object looks higher above the ground than it really is. But if the temperature goes down as you go up in the atmosphere, you might get an inferior mirage – an object looks closer to the ground than it really is.
When you see a highway mirage – it looks like water on the road ahead of you – you’re seeing an inferior mirage. That’s because an asphalt surface is much hotter than the air above it, as you’ll realize if you try to walk across it barefoot. The very hot road and the cooler air above creates a mirage. The image of something higher up is refracted downward onto a road surface – to create what looks like a pool of water on the road ahead. This mirage is really an image of blue sky on the horizon.
So, on a long car trip, you sometimes see what looks like water on the road ahead. When you reach it, it’s gone. That’s a mirage.