MENIERO.IT | Sky Photography

Optical illusion of mirages

Our sense of sight can be deceived in such a way as to make us lose the right points of reference in rationally recognizing a real phenomenon. We are talking about the optical illusions of mirages, traps that nature sets for us by making us wrongly perceive the positions or the sizes of physical objects.

Since ancient times optical illusions have aroused strong emotions ranging from fear to dismay, from wonder to amazement. Mirages, precisely because of their mystery, have fascinated humankind more than any other optical manifestation: through their expressive power they leave in our minds impressions that astonish and enchant.

Today science is able to provide a rigorous interpretation of atmospheric phenomena and fear fades away, giving way to surprise and wonder.

The main optical illusions can be either physiological or physical in nature. In the first case these are deceptions arising from cognitive processes. In the second, these are distortions resulting from real curvatures that light rays undergo when they pass through inhomogeneous layers of the atmosphere (atmospheric mirages), or when light coming from a distant galaxy is bent by the gravity of a massive object along the line of sight (gravitational mirage).

Atmospheric mirages in history

The first scientific study of mirages dates back to 1798, and is attributed to the French mathematician Gaspard Monge, who observed them during his scientific expeditions in Egypt. Monge, following Napoleon, observed that marching battalions, when seen from afar, seemed to float in the air, distort and stretch; the French army thus appeared larger than it really was. The scientist noted that this peculiar atmospheric phenomenon changed the size of the many French camps in the desert, turning them into real fortifications.

He understood that studying this unknown physical phenomenon could help Napoleon’s military needs; he therefore pointed out to the French generals Berhollet and Desaix that the desert could make their weapons deployed in the field invisible, or multiply their number.

The detail that impressed Monge the most was that, during these apparitions, the desert took on the shape of an elusive lake of water. When he returned home to describe the phenomenon, he coined the term “se mirer”, which can be translated literally as “it reflects”; later the term was translated into mirage (English) and into miraggio (Italian).

Monge’s experience influenced Mediterranean popular culture, which tends to associate the phenomenon only with the hot, dry regions of deserts. Mirages occur more often over sea expanses and in Arctic regions and, not by chance, Asian observers (not influenced by Monge) gave the phenomenon names with the water element in the etymon; for this reason in Japan the terms nigemidu and kagenuma were adopted (respectively flooding and false stretch of water) and, in ancient Chinese, the expression shuiying indicated a fake image on the water.

Some writers and scientists, including Monge, put forward an interesting theory about the biblical passage of the Red Sea. This theory holds that the Egyptian army, pursuing the Israelites, refused to cross its waters because it was swayed by a mirage. The event was explained as follows: the Israelites looked at the Red Sea and saw that it was made up of a very vast expanse of water, but when they reached its shores, they found no trace of it and believed the water had withdrawn; in reality they had previously seen a mirage. After crossing the ford they turned around and saw the Egyptian army submerged by the water, right at the same point where they had already passed; they were seeing a mirage for the second time because the desert appeared wrongly flooded. The Egyptians, once they too reached the shores of the Red Sea, saw the Israelites moving over the expanse of water and stopped, believing the fugitives had supernatural powers. Pharaoh’s army did not understand that it was observing the mirage of a false sea expanse. A similar experience can be lived daily by watching in summer the asphalt that seems to liquefy at the points farthest from the observer (see inferior mirages).

From the example described it follows that scientific ignorance can be the cause of a wrong interpretation of a mirage. Over the centuries sailors, merchants and explorers have been deceived by Nature which, by fooling the sense of sight, made them lose the right points of reference. Even today, many occasional observers even believe they have problems with their eyesight when they observe such phenomena.

The formation of atmospheric mirages

Physical optical illusions belong to the family of photometeors and are produced by temperature differences between the various layers of air and between these and the Earth’s surface. Under particular conditions, objects and landscapes, located at a considerable distance from the observer, can produce virtual images and be seen distorted, doubled or inverted, even by different viewers, many kilometers apart.

Mirages can mainly appear in different ways: “inferior mirages”, “superior mirages”, with three refractions, with more than three refractions (Fata Morgana).

In the case of inferior mirages, the lowest atmospheric layers, in direct contact with the ground, warm up more than the upper ones; consequently, if no rising currents form, a real carpet of unstable warm air is created over the ground and thus a vertical variation in air density such as to produce refraction and reflection phenomena. The light rays that meet this thickness of air do not propagate in a straight line, but curve upward, producing a virtual image inverted downward.

To understand the mechanism well, one must grasp the concept of refraction: when light strikes the boundary surface between two different air masses, it splits into two parts, one part is reflected with a reflection angle equal to the angle of incidence, the other part continues to propagate beyond the boundary surface. If the incident ray is not perpendicular to the boundary surface, the ray that enters the second material has a direction different from that of the incident ray. This ray is called the refracted ray and shows one of the following behaviors: when light passes from a medium with a lower refractive index to one with a higher refractive index (for example from air to water), the refracted ray approaches the normal (it propagates in water); when light passes from a medium with a higher refractive index (water) to one with a lower refractive index (air), the refracted ray moves away from the normal and propagates in air. The most frequent example of an inferior mirage is the one that occurs in summer over asphalt stretches, where even 70°C can be reached in the first 15 cm of height from the ground. From afar, it seems that the road is “wet”: but what you see is the sky reflected by the layer of warm air above the asphalt. An example of these mirages is the one described by Latham W., which occurred in the English Channel, in which the French coasts appeared incredibly close. The ancient Nordic peoples called this kind of mirage hillingar, which can even show sailors distant coastlines, located well below the horizon. According to some scholars, hillingar contributed to the discovery of Iceland and Greenland.

The superior mirage, instead, is generated when the air layer is colder than the ground and denser than the upper air layers. The visible effect is reversed compared to the inferior mirage.

Lower mirages

Inferior mirage of an aircraft on takeoff roll

Thermal inversion line

Variation of mirages during the day: photos of the Torre della Meloria in front of Livorno taken from the beach of Marina di Pisa. The image on the left was taken in the morning at 08:30 and shows an inferior mirage. The central image at 12:30 records an inferior mirage of lower intensity. The photo on the right did not record any mirage and was taken at 19:30.