Thirty six degrees of temperature, an endless straight road and in the distance we make out a puddle of water. However, as we get closer we discover that it does not really exist, that it is simply an optical illusion, a mirage. Physics has played a trick on us.
To understand why this phenomenon occurs, first of all, we must know that our brain distinguishes objects thanks to the light that comes from them. For light to reach the photosensitive cells (cones and rods) of the retina, it has to pass through the various transparent media that make up the ocular optics (cornea, aqueous humor, lens, and vitreous humor).
The retina translates light stimuli into impulses that eventually reach the brain to be reinterpreted. In a mirage our brain transcribes reality incorrectly.
Upper and Lower Mirages
This phenomenon is an optical illusion that is generated by the difference in temperature at which the different layers of the atmosphere are found. When the light does not change medium -there is the same density- it moves in a straight line. This is what happens, for example, when light travels through air.
However, if a part of the air changes density, changes in the refractive index occur, which causes the light to bend. It is the effect that we have been able to observe when we introduce a pencil into a glass of water.
Well, this phenomenon is what occurs in the desert, where the temperature of the layers closest to the ground increases, so that they become more dense. Due to this lack of uniformity, the light that comes from the sky seems to also emanate from the ground and this is interpreted by our brain as the reflection of the sky on a puddle or a small lake of water.
Mirages occur when temperatures fluctuate, whether they get colder or warmer. In the first case, the so-called superior mirages take place, that is, optical illusions that occur when the air that is closest to the surface cools more than the air that is at higher levels.
In contrast are the lower mirages, which are the most common and are the ones seen in deserts and on highways during very hot days.
The Fata Morgana Effect
According to Arthurian legend, Morgan le Fay was the half-sister of King Arthur, a fairy capable of changing shape – from Italian fata, fairy. For this reason, her name has come to refer to a superior mirage produced by thermal inversion.
This effect is relatively common in the Strait of Messina, where locals see islands, cliffs, ice floes, or even ships floating on the sea on the horizon, giving them a ghostly appearance. Generally the Fata Morgana effect is visible in the morning after a cold night.
For centuries this phenomenon was linked to navigation in certain parts of the globe and with certain atmospheric conditions, since the scientific reasons were not known, it is not difficult to imagine that they caused panic among the crews.
In Homer’s ‘The Odyssey’, reference is made to certain marine legends that state that when men have been on the high seas for a long time they can have visions, sometimes their own and in other cases caused by the gods.