In the middle of summer, on an asphalt road heated by the sun, it's common to spot what looks like a patch of water in the distance. Drivers and cyclists experience this: it appears as a shiny puddle right ahead of them, but as they approach, it vanishes as if by magic. This phenomenon has a name: the mirage.
A mirage isn't a hallucination but rather an optical effect. It occurs when the light we see bends as it passes through layers of air at different temperatures. In summer, the sun heats the road surface, which in turn warms the air just above it. The result: a layer of very hot, less dense air forms close to the ground, while the air higher up is cooler and denser.
The lake you see in this photo isn't real. It's a mirage like those sometimes seen in deserts.
Photographed in Primm, Nevada, on April 4, 2007. Light travels at slightly different speeds depending on air density. When it passes from a denser air layer to a warmer, less dense one, it gradually curves. This is called refraction. On an overheated road, this bending is significant enough that light from the sky is deflected toward our eyes.
Our brain then interprets these light rays as if they came from the ground. Since the light carried is from the sky—often blue or bright—we perceive it as a reflective surface, resembling water. So it's not a puddle but a piece of the sky "projected" by the road, following the laws of optics.
This type of mirage is called an inferior mirage because the image appears to form below the real object (in this case, the sky). This phenomenon is very common in arid areas, on asphalt roads in summer, or even above desert dunes. Travelers on foot or horseback have often mistaken these reflections for actual water sources, which is why the word "mirage" is associated with the idea of a deceptive illusion.
Mirages aren't limited to summer roads. In mountains, at sea, or in polar regions, other types can be observed, such as the superior mirage, caused by inverted air layers (warmer above, cooler below). This type can create the illusion of boats or coastlines floating in the air.
In reality, seeing a mirage proves that our eyes and brain are working perfectly... but that the physics of light doesn't follow the same path as our intuition. The phenomenon depends on various factors: ground temperature, solar radiation intensity, air humidity, and even the color of the surface.
To observe it easily, just drive on a road in the middle of a summer afternoon and watch the horizon. The "puddles" appear and disappear as you move forward, as if retreating ahead of you. This is the very signature of a mirage: an image that exists only in a precise position, vanishing as soon as the viewing angle changes.