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Fata Morgana and Mirages July 22, 2012

Posted by pswanso233 in Astronomy, physics.
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I was scanning through the archives of the Astronomy Picture of the Day, and I saw this one which I thought looked really cool:

This is the cool picture I saw cruising through the APOD archives.

(http://apod.nasa.gov/apod/ap120415.html)

Interested, I looked up Fata Morgana and learned that it was a type of superior mirage.  Not knowing what a superior mirage was, I had to understand what causes mirages and what the difference between an inferior and a superior mirage was.

A mirage is a real optical phenomenon, rather than a hallucination.  Mirages can actually be photographed, whereas hallucinations cannot be.  Mirages are caused by temperature differences in the Earth’s atmosphere.  It’s here that I should probably introduce Snell’s Law and refraction, which is the bending of light through different materials.  Every transparent subtance has what’s called an index of refraction, which is the ratio of the speed of light in vacuum to the speed of light in that substance.  A high index of refraction indicates that light travels very slowly through the substance, whereas a low index means it doesn’t slow down much.  For example, the index of refraction of water is 1.33.  This means that light travels 1.33 times slower through water than it does air or vacuum.  This is why a pencil looks bent if you put it in a water filled beaker while still allowing part of it to be in the air.

A pencil looks bent when placed in water at an angle to the vertical because of refraction.

(http://library.thinkquest.org/27066/lightrays/nlrefraction.html)

A high index of refraction also means light will bend more if it travels through that substance.  So how exactly does this apply to mirages?  Cold air is denser than warm air, so light has a harder time going through it; therefore its index of refraction is higher.  If light rays from a distant source travel from cold air to hot air, they will bend away from the direction of the temperature gradient.  As these light rays reach your eye, your brain traces it as though they came from a line straight ahead, similar to your eye interpreting a virtual image through a convex lens.

An inferior image is a type of mirage where an image appears to be below a real object.  A common example would be a desert mirage, where the viewer thinks that there’s an oasis on the horizon.  This is caused because sand tends to heat up quickly, so the air around the sand is hot and the air above it is cooler.  The image you’re actually seeing is actually the sky, which is why it looks like water.

Inferior Image Formation

(http://www.islandnet.com/~see/weather/elements/mirage1.htm)

A superior image is the opposite case, where the image appears above the horizon.  This is caused by what’s called a temperature inversion, where hot air exists above cold air.  This tends to be more common at sea.

Superior Image Formation

(http://www.islandnet.com/~see/weather/elements/mirage1.htm)

Anyway, as I said before, a Fata Morgana is a special case of a superior mirage.  They can be seen from anywhere on Earth, but tend to be most common in Polar Regions and higher altitudes.

The special case of the superior mirage of a Fata Morgana occurs when the temperature inversion is high enough such that the light bends through it in such a way that the curvature of the light is higher than the curvature of the Earth.  The viewer should be present in an atmospheric duct, which is where light rays and other electromagnetic waves bend with the curvature of the Earth.  This is why these images tend to be rarer than other types of mirages.

A fata morgana off the coast of Canada

(http://www.sciencephoto.com/media/420314/enlarge)

          A Fata Morgana usually looks very bizarre, and can produce stacked images on top of each other.  They can also change rapidly, as if the temperature gradients change in such a way that the light no longer bends with the curvature of the Earth, they become regular superior mirages and don’t necessarily appear on the horizon anymore.

Another example of a fata morgana

(http://www.fedbybirds.com/2009/03/fata_morgana_in_finland.html)

           Fata Morgana is named for “The Fairy Morgana”, Morgan le Fay, who opposed King Arthur and Queen Guinevere in Arthurian legend.  She was a sorceress who had affairs with some of Arthur’s knights and was also Arthur’s half-sister.

One other cool and (sort of) funny I learned is that, back in the early 1900’s, some explorers found what they called the “Crocker Land”, which was a supposedly large island that existed between Canada and Greenland.  A very expensive team was sent to survey the island, but the mission cost over $100,000 (a huge sum at the time), because the island they saw was, in fact, a Fata Morgana.  They were even warned by some of the natives of Greenland that it was an illusion, but they pressed on anyways and were unable to explore the Crocker Land.

There is also some interest that perhaps a Fata Morgana contributed to the sinking of the Titanic:

http://www.dailymail.co.uk/sciencetech/article-2130598/False-wall-water-created-Fata-Morgana-mirage-hidden-iceberg-Titanic-late.html

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