Optical duct casts land images afar

Nov. 1, 2000
The sighting of low, barren islands, or skerries, off the northwestern coast of Iceland was the first indication to ninth-century Europeans that something other than open ocean lay in that direction.

The sighting of low, barren islands, or skerries, off the northwestern coast of Iceland was the first indication to ninth-century Europeans that something other than open ocean lay in that direction. The event most likely led to the discovery of Greenland, which looms beyond the position of the sighted islands and 285 km away from Iceland itself. In all the centuries following, however, the skerries were never found, and for good reasonthey do not exist. Now, a researcher at the University of Manitoba (Winnipeg, Canada) may have solved the mystery of the disappearing islands.

Waldemar Lehn hypothesizes that an atmospheric formation called an optical duct made the coastline of Greenland itself visible for hundreds of kilometers, although in an indistinct, flat-topped form. The resulting image would appear as a band of uniform height and varying intensity that, when seen through the scatter of the atmosphere, would look like skerries.

An optical duct comes into being when a temperature inversion in the air tens of meters above the water produces a sharp gradation in refractive index. Such a gradation serves as an upper barrier confining light by total internal reflection. The confinement of light below occurs simply as a result of the curvature of the Earth and thus the optical duct. In other words, the propagation of light through an optical duct is akin to "whispering-gallery" propagation in a microdisk cavity, only on a vastly larger scale.

For one of his models, Lehn chose a situation in which a ship was positioned 220 km away from Greenland. He modeled both a full optical duct (extending all the way from the ship to Greenland) and a partial duct (extending from the ship to a point 120 km away from the ship). The full duct was bounded by a 5.5°C temperature inversion 33 m above water level, while the partial duct was edged by a 9°C inversion 50 m high; in fact, the precise temperature profile of the ducts is not important, says Lehn.

Both simulations resulted in skerrylike images. In the case of the partial duct, the light path from Greenland spanned a distance of 100 km before entering the duct, bouncing twice off the temperature inversion, and reaching the ship. By hastening the Norse discovery of Greenland, such a phenomenon surely hastened their later discovery of North America, as well.

About the Author

John Wallace | Senior Technical Editor (1998-2022)

John Wallace was with Laser Focus World for nearly 25 years, retiring in late June 2022. He obtained a bachelor's degree in mechanical engineering and physics at Rutgers University and a master's in optical engineering at the University of Rochester. Before becoming an editor, John worked as an engineer at RCA, Exxon, Eastman Kodak, and GCA Corporation.

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