Mirages are caused by the bending of light waves through refraction, which occurs when light passes through different materials. Inferior mirages occur when heat radiates from the ground and bends light, creating a double image that looks like a reflection in water. Superior mirages occur when a cold air mass is under a warm air mass, and objects appear taller than they are. Superior mirages are visible at long distances due to the curvature of the Earth.
There are two types of mirages: inferior and superior. Mirages are caused by the refraction of light, which is the bending of light waves. Refraction occurs when light passes through one type of material into another. For example it can pass through the air and enter the water. Light can also be refracted as it passes through an area of warm air into an area of cooler air because cold air has a higher density than hot air.
In an inferior mirage, which is the most common type, an object appears to be present as if it were both the real object and its reflection in a pool of water. When the ground is very hot, the heat radiates from the ground and warms the air directly above it. As light passes through the cooler air above into the warmer air below, it bends and creates a mirage. The mirage looks like an object reflecting in water because some of the light would usually go to the ground, but it is bent and instead rises to the eyes creating a double image. Inferior mirages are commonly seen on paved roads during hot days and often look like puddles of water on the road surface. This is the mirage in the desert, the scenario most frequently associated with mirages.
A superior mirage appears on the horizon because a cold air mass is underneath a warm air mass. It usually appears on ice or very cold water. In this type of mirage, the object appears to be much taller than is logical. Examples of this type commonly seen by sailors are images of floating islands and land masses where there are none. Superior mirages are most commonly seen in the polar regions due to the large amounts of ice and very cold water.
Superior mirages are visible at long distances because the Earth is round. If the Earth were flat, the light that was bent down would reach the ground very close to where it was bent, and the mirage would only be visible at a very close distance. An interesting example of this occurred in 1596, when a ship searching for the Northwest Passage became trapped in ice in the far north of the Atlantic Ocean and the crew had to stay there through the winter. Because the sunlight was refracted in a curve that follows the curvature of the Earth, the crew saw the sunlight two weeks before the very long midwinter night actually ended.
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