Mirages are images caused by atmospheric refraction.
Let’s break that information down:
Light has different speeds depending on the medium. It travels the fastest in a vacuum, and still pretty fast in air as well. When it has to go through glass or water, however, it slows down significantly. Whenever light changes from one medium to another at an angle, one side of the “ray” is slower then the other side. The light will bend and we call this refraction.
Refraction in water
Rainbows are caused by light hitting water drops and bending in the same way a prism works. Refraction can easily be observed in a glass of water. A straw in a glass of water will look bent where it breaks through the water. These soldiers also suffer from a case of refraction:
Refraction in glass
Lenses work thanks to refraction in glass. Glasses, microscopes, binoculars and cameras all use refraction. The shape and thickness of the lens is how we can control the amount of refraction and therefore the enlargement of the image..
Refraction in air
The refraction of light in air depends on the density of the air, and therefore on the air temperature and pressure. A very good example of visible refraction in air is a heat haze. On a hot summer day you can observe a typical haze above asphalt indicating the hot air mixing with the cold air. These are therefore often called “highway mirages”.
Inferior mirages
Highway mirages are inferior mirages. When a layer of hot air sits below cooler air, the light from the sky will bend upwards as soon as it comes close to the hot air. We see this often happening on asphalt or in the desert.
By bending the light all the way upwards, the image gets flipped upside down. In many cases this means that we get to see a blue reflection of the sky on the layer of hot air. Our brain will complete the illusion by concluding that it is water; the oasis in the desert, the wet patches on the highway or tarmac.
Superior mirages
A superior mirage works in the opposite way of an inferior mirage. There is a layer of cold air below warmer air and light will bend downwards when it reaches the colder air. This occurs often on the ocean or on ice.
When light gets bent down, we will perceive the image to sit higher than it actually is. A common example is a ship or an island that seems to float a little above the water.
Fata Morgana
The illusion that plagued people in legends as well as in real history is the Fata Morgana. Usually described as something mysterious that can’t be reached, this optical illusion is actually caused by several layers of warm and cold air.
We will then see a combination of superior and inferior mirages in segments op top of each other. The result is often unrecognizable and looks like mountains, islands or even buildings.
In history, some phantom islands have been put on maps, but even in modern days people get tricked by Fata Morgana’s and report seeing UFOs.
