In summer it is impossible not to notice it: walking along a road the asphalt in front of you looks wet. However, when we reach the area dotted with a series of bright and flickering puddles we realize that it is not, and that this effect continues to recede on the following stretch of road. This happens because we are in front of a miragethat is, a natural optical illusion that is created when for some reason there is a deviation of light as it passes through areas of air with different densities. But from a technical point of view, why does the road look wet? Simply put, the asphalt appears wet because it is actually an upward reflection of the sky above: the “wet” effect is then amplified by the shimmering of the air due to the heat emitted by the asphalt itself.
The ultimate cause of this optical phenomenon is the thermal gradient above the asphalt: on summer days, asphalt absorbs the heat of the sun and is much warmer than the surrounding air. For this reason, the air temperature decreases rapidly with height from the ground. Now, the air temperature influences its ability to deviate the trajectories of light rays. In technical jargon, it is called refractive index: the higher the refractive index of a material, the more the light rays change direction when they enter that material. The refractive index of hot air is lower than cold air, so as the light coming from above descends, and therefore encounters increasingly warmer air, the trajectory of the rays becomes less and less inclined with respect to the horizontal. If the thermal gradient is sufficiently high, the phenomenon of total reflection: the rays are deflected so much that they point upwards again before hitting the asphalt. At this point the blue light from the sky reaches our eyes and this gives us the illusion of looking at a puddle of water. In reality we are looking at the image of a piece of sky reflected by the air!
However, warm air is less dense than cold air, so it tends to rise upwards. And this is precisely what vertical difference in density to set in motion vertical motions that trigger turbulence that makes the air appear to tremble. This effect increases the impression that we are looking at a puddle of water.
A similar effect occurs in the famous mirages in the desertwhen you think you see puddles of water. Even then, it’s still a patch of reflected sky you’re looking at.
This kind of mirage, the “lower mirage“, it occurs if there are two layers of air at very different temperatures, and since the light is totally reflected, the objects that we see on the ground appear upside down (as in this case) or lower than where they really are, but it is not the only one. There are also superior, inferior, lateral and “combined” mirages, which are rarer to see. The latter is the case of a well-known optical phenomenon that can be observed over the Strait of Messina, the “Fata Morgana”, and is a combination of inferior and superior mirage that makes objects on the horizon appear as if they were blurry turrets.
