Visual curiosities: Mirages and Fata Morgana

27/09/2024

We have all experienced this phenomenon at some point. Mirages occur due to certain situations and atmospheric conditions that make us see illusory images.

We are driving on the road in the summer, at midday. It’s hot, cloudless, and it hasn’t rained in weeks. At the end of a straight road, large puddles appear in the distance. The sky and even the cars driving further away are reflected in them. But when we get closer, the puddles vanish: the asphalt is completely dry.

A mirage is the most common form of deceptive perception: a natural optical phenomenon where the sky and distant objects appear reflected on a surface that seems liquid. Light rays are deflected by temperature changes or gradients: hot air next to the asphalt causes refraction and “bends” the rays, creating a lower image that “reflects” the sky as if there were water. This is known as an inferior mirage, typical of deserts, where the “false” image appears below the real object.

There’s also an opposite effect: an image floating “suspended” above the horizon. This is a rarer superior mirage: it occurs in cases of thermal inversion, when there is a colder air mass layer below and a warmer one above. It usually appears in polar regions or at other latitudes over a cold body of water with a layer of warmer air above it. I. The typical effect is of a distant, very high and abrupt coastline, and sometimes with ships or coastal cities seemingly floating in the sky.

For a superior mirage to occur, the observer must be at the height of an atmospheric “duct”, between the cold air below and the warm air above. Here the rays of light bend parallel to the curvature of the Earth explaining why objects like the rising sun or distant ships, located beyond the geometric horizon, can be seen. Depending on their distance and the optical conditions, these objects might appear upright or inverted, sometimes even switching between the two.

Unlike the so-called “optical illusions,” which despite this name are usually perceptual in nature, a mirage is a real optical phenomenon and can be photographed, even if the mind’s interpretation of it can be illusory. It is striking that, while the etymology of Spanish refers to the specular effect, that of other languages ​​such as French or English (“mirage,” from Latin mirare) points to the fascination of the gaze itself. Alongside mirages, terms like trampantojos, pareidolia, or hallucinations—though not exactly perceptual illusions—help us define and understand these effects. We’ll explore these concepts in future installments.

The Fata Morgana phenomenon

A complex variant of the superior mirage is known as “Fata Morgana” (named after the Italian version of the sorceress and half-sister of King Arthur, famous for her shape-shifting abilities), based on sightings in the Strait of Messina. This mirage changes rapidly with atmospheric fluctuations, altering parts of the image, stretching and compressing, upright and inverted.

Prof. Rafael I. Barraquer, Medical Director of the Barraquer Ophthalmology Centre