A  mirage is an  optical illusion that is created when light rays are refracted (bent) as they pass through adjacent layers of air of different temperatures.

For example, suppose you are in desert where the Sun has warmed the sand, and this warm sand has heated up the layer of air immediately above it. Colder layers of air will lie above this hot thin layer.

Mirage of Nonexistent Lake in Mojave DesertBecause hot air is less dense than cold air, light moves more quickly through hot air than through cold air. The change in the speed of light as it passes across the boundary between the air layers of different density causes the light to bend.

Imagine that you are standing on a sand dune somewhere in the desert. A group of palm trees is on a dune several hundred yards away from you. A layer of hot air lies between that dune and the dune on which you are standing.

You will see two groups of palm trees, even though there is really only one.

One image will come from light following a direct route across through the air.

The second image will appear upside-down below the first. It will have been created by light that was refracted and then traveled to your eyes.

This light will have traveled diagonally downward from the palms into the heated air blanketing the desert and then been refracted upward, meeting your eyes from below, as if a mirror were lying on the desert floor showing an inverted reflection of the palms.

At the same time light from the sky is also refracted by the hot shimmering air, giving the impression of a sheet of water in which groups of palms are standing, the real ones upright, the mirages upside down.

There are many stories about wanderers in the desert being driven insane by this type of mirage.

The puddles of water that we sometimes see in the summer on highways and other hot surfaces are also mirages.

They are light from patches of sky refracted by the heated air above the hot surface.

North America's southwestern deserts are also famous for their mirages.

Polar mirages occur when the air immediately above the ground is very cold and there is a warmer layer above it. The observer may see images of distant objects dis¬placed upward in the sky. These mirages often present double images.

For example, when ships or icebergs are floating in a calm sea, they create a mirror image in the water below them.

A distant observer often sees both the image of the object and its reflection in the warm upper layers of the air.

Conditions for the inverted type of mirage sometimes occur above Paris. Then the Eiffel Tower appears to balance an upside-down image of itself.

Mirages do not always show the true shape of an object.

Images may be enlarged, contracted or distorted, depending upon the position and composition of the refracting air layers.

A palm tree may appear to be a blade of grass. A small fishing hut may look like an enormous palace.

Sometimes you may be able to see a mirage of an object that lies around a corner. This can happen when a cold, refracting layer of air is created alongside a vertical surface, such as an icy cliff wall. Light rays may then be bent by refraction to curve around the base of the cliff.

Fata Morgana

A fata morgana is a complex mirage that combines many different types of mirages - upright, inverted, enlarged, diminished and distorted multiple images.

The Strait of Messina, between Italy and Sicily, has a well-known fata morgana.

For a fata morgana to occur, the sea must be fairly warm, heating the layer of air in contact with it, and a second layer of warm air must exist at some higher altitude, so that a cool layer of air is sandwiched between two warm layers.

The cold middle layer produces a double mirage and acts as a cylindrical lens that magnifies vertical dimensions, so that distant objects may look like towers or spikes.

A fata morgana is usually preceded by a strange-looking cloud in the sky.

When the air above the Strait of Messina is hot, and the water is calm, such a cloud shimmers with the image of a splendid harbor town.

Soon a second town may appear, piled on top of the first, and yet a third, all with shining towers and palaces.

Sometimes it seems as if houses lie below the water.

People can be seen strolling through the streets in billowing white garments.

No one is certain where the images of the fata morgana in the Strait of Messina come from.

Some believe that the mirage is a refracted image of the Sicilian harbor of Messina.

Others say the mirage is really the image of a strip of coastline, whose trees and stones have been magnified and distorted to suggest palaces and towers.

Still others think that the beautiful town in the mirage is really the refracted image of an obscure Italian fishing village.

The term "fata morgana" is sometimes used to refer to any mirage.

Mirages in History

The American explorer Roy Chapman Andrews once saw strange animals that resembled huge swans wading in a lake in the Gobi Desert.

From a few hundred yards away, they seemed to be gigantic creatures that walked on stiltlike legs nearly 15 feet long.

As Andrews came closer to the lake, the water shrank and its inhabitants changed shape. The plump giant swans became slender antelopes grazing peacefully on sparse desert vegetation.

A layer of hot air had produced the impression of water, and the unevenness of the layer had grotesquely distorted the images of the antelopes' bodies.

The army of Napoleon encountered mirages in Egypt in 1798.

Confronted with upside-down landscapes, blades of grass that turned into palm trees and lakes that disappeared, Napoleon's troops are said to have fallen on their knees and prayed to be saved from the impending end of the world.

The French mathematician Gaspard Monge was a member of Napoleon's expedition. He came up with a scientific explanation for the mirages.

During the Anglo-Turkish battles of World War I, a mirage forced the British artillery to stop firing.

The mirage appeared before the artillerymen's eyes and completely masked the enemy's position. The British commander's report on the shelling says, "The fighting had to be temporarily suspended owing to a mirage."

Near the North American coast during the World War I, a German submarine commander looked through his periscope and saw the skyscrapers of New York  hanging in the air above him. The city looked like it was about to plummet upside-down into the water. The baffled submariner is said to have retreated hastily out to sea.

In 1912, during the South Polar Expedtion of Commander Robert Falcon Scott, men returning to the coast from an inland trek saw the support ship Terra Nova hanging in the sky in double form.

There was an inverted image of the sailing ship, with an upright image above it, and smoke was drifting in opposite directions from the respective cookhouse chimneys. Although the ship itself was hidden by intervening hills, its mirage showed that all was well on board.

Crocker Land

The American polar explorer Robert Peary once reported the existence of an unmapped Arctic mountain range, which he named Crocker Land.

Many explorers went to the Arctic in search of these mountains, but they never found them.

Finally, the American Museum of Natural History in New York City donated $300,000 to mount a scientific expedition into the area.

Donald MacMillan led this expedition in 1913.

Where Peary had seen mountains, MacMillan's party found only an icy waste. Where, according to Peary there were deep and wide channels, floes of pack ice threatened MacMillan's ship.

Crocker Land eventually did appear, but it was 200 miles farther west of where Peary reported it.

MacMillan sailed as far as possible through the ice floes toward Crocker Land.

Then he dropped anchor and set out on foot over the ice. However, when Peary and his team tried to approach the mountains, the mountains receded. When they stood still, the mountains stood still.

Eventually, the men reached a valley enclosed by mountains on three sides.

When the Sun sank below the horizon, the surrounding peaks and foothills disappeared.

The men found themselves on a vast, flat expanse of ice that surrounded them in all directions as far as they could see.