Our brain deceives us. But not on purpose, it is just a fault when optimizing processes. The brain is ready to respond equally to similar stimuli automatically, ‘without thinking’. This saves time when interpreting the reality in which we live, but there is a problem: it makes us believe things that do not exist when reality is strange, and optical illusions or mirages are a good example of it. So what happens when we see a mirage?
The physics of illusions
First thing to know is that mirages are not hallucinations, or inventions of our brain; it is just a misinterpretation of reality. Proof of this is that, unlike ghosts, pink elephants or sasquatchs, mirages can be photographed and even searched. The key is how light travels and how the heat affects the air molecules.
The path followed by the light depends on the properties of the medium in which it moves. For example, when light passes from water to air its direction changes, whereas when traveling without changing medium it goes straight. The density of the medium affects the movement of the light; well, technically it is due to the atoms and their disposition, but for us density is a sufficiently good approximation.
The funny thing happens when density changes smoothly as when there is cold air at the bottom and hot air at the top. This always happens, but only on large scales. When it occurs at small scales, the effects become visible and we find the mirage, which is nothing but light that is where it should not be. Like in a very hot day when the asphalt is more heated of what the sun heats the atmosphere.
What the brain interprets
As the light goes through layers of less and less dense air it keeps bending until the light that comes from the sky begins to travel from the bottom to the top; as you can see in the image. When this happens, our brain receives images from the sky as if they came from the earth and automatically decides that that blue light we observe is on the ground.
And of course, something blue, which moves (due to convective air currents)… our brain quickly assimilates it to water. Mirage for sure to come! A natural optical illusion that occurs not only in the desert or hot asphalt, but also at the sea: the brain now believes we see air and flying boats; or inversely so the boat also appears on itself.
It is the price we pay so that every time we look at something our brain understands it almost immediately, without having to analyze what is good really. The fact of seeing water in the middle of the desert is probably not ideal, but seeing boats flying could be quite interesting!