MIRAGES

Mirage

We generally hear interesting stories about weary travelers trudging across a seemingly never-ending expanse of desert to what they think is a lush oasis or people traveling on a hot afternoon notice a puddle of water which melts into thin air when approached. This optical phenomenon is known as mirage. The term mirage originates via the French word ‘mirage’ which originates from the Latin mirari, which means ‘to look at, to wonder at’.

Fata Morgana

A mirage is generated by two layers of air at different temperatures. Since cold air is denser than hot air, the boundary between the layers can refract (bend) light, especially if the light hits the boundary at an extreme angle. This effect can be commonly seen on asphalt roads during the summer. The black color of the road gets the air above it hot very quickly, creating a sharp temperature gradient in the air – the first ingredient of a good mirage. This is often referred to as a highway mirage.

Inferior Mirage

Mirages can be categorized as “inferior” (meaning lower), “superior” (meaning higher) and “Fata Morgana”, one kind of superior mirage which appears above the water. The phrase comes from the Italian version of the name of the sorceress Morgan Le Fay.   An inferior mirage is called ‘inferior’ because the mirage is located under the real object. Inferior images are not stable. Hot air rises, and cooler air being denser descends, so when the layers mix, it gives rise to turbulence. A superior mirage occurs when the air above the line of sight is colder than the air above it. Superior mirages are in general less common than inferior mirages, but when they do occur, they tend to be more stable. These mirages are quite common in Polar Regions, especially over large sheets of ice with a uniform low temperature.

Superior Mirage

Researchers from the University of Texas at Dallas have created a working cloaking device which has the ability to make objects disappear. The novel design makes use of sheets of carbon nanotubes (CNT) which has unique properties to conduct heat and transfer it to surrounding areas that makes it an ideal material to exploit the so-called “mirage-effect”. Just like a mirage, this steep temperature gradient causes the light rays to bend away from the object concealed behind the device, making it appear invisible. The research results also provide useful insights into the optimization of nanotube sheets as thermo-acoustic projectors for loud speaker and sonar applications, where sound is produced by heating using an alternating electrical current.

The optical phenomenon of a mirage of an astronomical object is most commonly observed for objects like the Sun, Moon, the planets, the bright stars and very bright comets in which light rays are bent to produce distorted or multiple images. The most commonly observed are sunrise and sunset mirages.

Desert Mirage

For those who have never spotted a mirage, there is no need to trek to the desert to see a mirage: they are very common on roads. The most spectacular mirages occur in wide expanses of flat land as too many hills, dips or bumps will prevent the refracted light from reaching your eyes.