What Causes A Mirage?

Mirages appear in arid environments like deserts.
Mirages appear in arid environments like deserts.

The optical phenomenon called mirage occurs naturally, and it is as a result of the bending of light rays to create a displaced image of either distant objects or the sky. The term's origin lies in the French word mirage and the Latin mirari which translates to “to wonder at” or “to look at.” Mirages fall either into the superior, inferior or “Fata Morgana” categories. A mirage can be caught on camera since it is a real optical phenomenon.

Inferior Mirage

The inferior mirage commonly occurs in the desert, and it appears as a water body in a distance. The mirage is termed as inferior since it lies under the real object. The object, in this case, is the blue sky or another distant object in the same direction. The mirage leads the observer into seeing a bright and bluish area on land in the distance. The light rays that originate from a distant object travel through almost the same air layers and they are also bent over nearly the same amount. Rays traveling from the top of the object will thus reach lower than the rays from the bottom. The image is normally upside down making it seem like the sky image is a water puddle serving as a mirror. Inferior images are also not stable. It is characteristic of hot air to rise and cooler air, on the other hand, to descend which causes the layers to mix and turbulence to emerge. The image will subsequently be distorted accordingly, and it can be horizontally or vertically extended or vibrating. Several temperature layers can cause some mirages to mix creating double images. The fake water or hot-road mirage is a common example of this type of mirage. Inferior mirages can also occur over lakes or oceans by virtue of them being extensive and flat. The IJsselmeer and Markermeer lakes are some of the water bodies where inferior mirages are common.

Heat Haze

Heat haze is a kind of inferior mirage observed when objects are viewed through hot air. This phenomenon, also called the heat shimmer, can be seen by viewing objects through the exhaust gases expelled by jet engines or across the heated asphalt. Convection contributes to the variation of the air's temperature while the variation between the heated air at the road's surface and the cool air above results in a gradient in the air's refractive index. A blurred simmering effect is subsequently created which influences the capability to resolve objects. This effect is increased when the particular image is magnified using telescope lens or a telescope. Light originating from the sky at a slight angle to the sky is commonly refracted by the index gradient to look like the sky is reflected by the surface of the road. A person's mind observes this occurrence as a waterbody on the road because water reflects the sky as well, but this illusion fades as the person gets nearer. If the road is tarmacked, it appears like water or oil has been spilled. Tarmac and sand can get extremely heated if exposed to the sun facilitating the development of a mirage.

Superior Mirage

Unlike the inferior mirage, the superior one occurs when an object's image appears above the particular object. The phenomenon is observed when the air underneath the line of sight is colder in comparison to the air above it. This occurrence is dubbed as temperature inversion as the existence of cold air below warm air is a deviation of the atmosphere's normal temperature gradient. These types of mirages are rarer than inferior mirages, and they are more stable since cold air does not travel upwards, and warm air does not commonly move down. Superior mirages commonly occur in polar areas particularly above massive sheets of ice that bear a uniform low temperature. When the mirages appear at more moderate latitudes, they tend to weaker and less stable and smooth. A superior mirage can appear upside down or right-side up depending on the temperature gradient and the distance of the true object. The mirage is often observed as a distorted mixture of both up and down parts. The mirages can have a striking effect because of the Earth's curvature. Light rays can move large distances if their downward bending curve is nearly the same as the Earth's curvature. Although this phenomenon was first observed in 1596 at Novaya Zemlya, it was not explained scientifically until the 20th century. For every 69.05 miles, rays of light can move parallel to the surface of the Earth, and the Sun appears 1 degrees higher on the horizon. The inversion layer should, however, possess the right temperature gradient across the total distance for the event to be possible. In the same fashion, ships that are so far away may appear above or even on the horizon as superior images although they should not be sighted above the geometric horizon.

Fata Morgana

The term Fata Morgana is an Italian translation drawn from Morgan le Fay who is described as a shape-shifting fairy in legend and as the half-sister of King Arthur. The word is used to describe a complex superior mirage which forms with alternations of compressed as well as stretched zones, inverted images, and erect images. This mirage is also noted for its fast-changing ability. A Fata Morgana commonly occurs on cold days in polar areas, in desert regions, and over lakes and oceans, as well as on hot days. For this mirage to occur, the temperature inversion has to have enough strength such that the curvature of the light rays within it is stronger in comparison to the curvature of the Earth. The rays will bend and form arcs, and the observer should be within an atmosphere duct to sight the mirage. The Fata Morgana can change from a superior to an inferior mirage in a matter of few seconds depending on the continually changing atmospheric conditions. A camera captured 16 frames of the mirage of the Pacific's Farallon Islands which are not visible at sea level under usual conditions since they are situated below the horizon on a single day. The first 14 images reflect alternations of stretched and compressed zones which are characteristic of a Fata Morgana while the last two frames were captured around sunset when the mirage was not as complex. The image distortions and the bending of light can create striking results.


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