Mirage
A mirage is an optical illusion resulting from the refraction of light in the atmosphere, primarily occurring when light rays pass through layers of air with differing temperatures and densities. This phenomenon is most commonly observed on hot days, where the ground heats the air immediately above it, creating a warm layer that bends light rays as they travel from cooler layers above. The most familiar type of mirage is the inferior mirage, often seen on roadways, where it appears that puddles of water are present, although they are not real. In contrast, superior mirages occur under different atmospheric conditions, typically when cold air lies below warmer layers, and can create images that appear above the actual horizon.
An advanced form of superior mirage, known as a fata morgana, can produce complex and distorted images, such as ships appearing to float above the water or entire cities seemingly suspended in the sky. These optical illusions can vary in appearance depending on the observer's distance from the mirage. Historical accounts suggest that fata morganas may have contributed to myths like the Flying Dutchman and may have played a role in maritime disasters, such as the Titanic's iceberg encounter. Overall, mirages illustrate the fascinating interaction between light, temperature, and human perception.
On this Page
Subject Terms
Mirage
A mirage is an optical illusion produced by the refraction of light beams in the atmosphere. Waves of light refract, or change direction, when they pass through layers of air with different temperatures and densities. A single beam of light would curve and bend when passing from an air layer of one density to a layer of another. Warm air, which is located close to Earth's surface, is less dense than the colder air of the upper atmosphere. This is why light refraction and, therefore, mirages commonly appear to human eyes on hot days.
People who witness mirages might believe they are seeing distant objects or structures that actually do not exist. One of the most frequently seen mirages is one of puddles of water on a stretch of road. The mirage is caused by light from the sky bending as it travels toward the ground. The human brain, however, automatically assumes that all rays of light travel in straight lines. This causes the brain to mistake the curved light for water because actual water on the road would reflect light from the sky. Other types of shimmering mirages may be seen in deserts, on airplane tarmacs, and above the waterlines of oceans or other large bodies of water, as all these locations feature convergences of cool and warm air layers.
Background
Mirages are directly caused by the interaction of light with air of different densities in different places in the atmosphere. The density of air is determined by the air's temperature. Cold air is more dense than hot air because it contains particles that collide with one another at slower rates. The particles in warmer air are more excited, due to heat, and thus collide with one another more often and with greater speeds. This makes hot air less dense than cooler air.
In addition to layers of air with different densities, sunlight is also required for mirages to occur. Humans are able to see their surroundings because of sunlight, which reflects off objects and shines out in every direction. Human eyes detect this reflected sunlight, which is why objects are visible to people.
Beams of sunlight that pass through layers of air that are all the same temperature, and that therefore have the same density, travel in straight lines. The human eye would detect this and not see a mirage on the ground. Light begins to bend when the layers of air it passes through on its way to Earth's surface are of different densities.
Photons, or particles of light, always travel from point to point using the path of shortest time. On a hot day, heat from Earth's surface rises off the ground and heats the layer of air just above it. A ray of light traveling from the sun to Earth passes through multiple layers of cool air before reaching this hot layer just above the ground. The light beam's photons begin taking a curved path to the ground as they encounter the warm layer of air because they seek the path of shortest time to their next point, even if this means taking a path of longer distance; the curve of the light beam, or its refraction, adds distance to the light's path, but the light's greater speed in the hot air compensates for this.
The human eye's detection of this refracted light allows people to see mirages. Sunlight directed toward a person does not bend because it reaches the eye before it can pass from cooler to warmer layers of air. Sunlight directed at distant points is refracted before reaching a person's eyes. The eyes' reception of both direct light and refracted light causes the person to see an image twice; this is because the human brain assumes light always travels in straight lines.
For example, on a hot day, someone may see an actual car on a road and a reflection of the car on the road's surface. People's brains relay that this reflection is caused by water on the road because water reflects whatever is above it. In reality, the reflection is only light that refracted as it passed into the layer of warm air above the road. This type of mirage is known as an inferior mirage, since it appears below Earth's horizon.
Impact
Superior mirages, conversely, appear above the horizon. They are formed when light passes through a reversal of the atmospheric conditions that cause inferior mirages. In the case of superior mirages, layers of cold air are located just above the earth's surface, below layers of warmer air. These circumstances may occur over extremely cold land areas, but they appear more frequently over large bodies of water such as oceans, since the layer of air directly above water is cooler than air layers higher in the atmosphere.
Beams of sunlight pass directly through the warmer, less dense air but bend as they enter the cooler, more dense air. The result is a superior mirage, a shimmering image of an object above the object's actual position in the distance. These images can be either blurry or perfectly mirrored, upside-down representations of the real object. An unusually complex and seemingly detailed superior mirage is known as a fata morgana.
Fata morganas appear differently to different people, depending on the distance between an observer and the mirage itself. The farther away a person is from the mirage, the taller the fata morgana will seem. Fata morganas are responsible for why people sometimes think they see distant ships floating above an ocean's waterline.
Some historians have posited that it was fata morganas that in centuries past led sailors to invent the legend of the ghost ship the Flying Dutchman. Other historians believe the crew aboard the Titanic in 1912 failed to see the iceberg that sank the ship because it was hidden in the distance by fata morgana effects. Some people have even reported seeing entire city buildings floating in the sky above real cities, phenomena that atmospheric scientists have confirmed could indeed be fata morganas.
Bibliography
Binns, Corey. "What's a Mirage?" Live Science, 29 Mar. 2021, www.livescience.com/32374-what-is-a-mirage.html. Accessed 19 Jan. 2023.
"False Wall of Water Created by 'Fata Morgana' Mirage May Have Hidden Iceberg from Titanic Lookouts until It Was Too Late." Daily Mail, 16 Apr. 2012, www.dailymail.co.uk/sciencetech/article-2130598/False-wall-water-created-Fata-Morgana-mirage-hidden-iceberg-Titanic-late.html. Accessed 28 Dec. 2016.
Harris, Tom. "How Mirages Work." HowStuffWorks.com, science.howstuffworks.com/mirage2.htm. Accessed 28 Dec. 2016.
Lucas, Jim. "Mirror Image: Reflection and Refraction of Light." Live Science, 1 Oct. 2014, www.livescience.com/48110-reflection-refraction.html. Accessed 28 Dec. 2016.
Lee, Jane J. "China's Floating City and the Science of Mirages." National Geographic, 20 Oct. 2015, news.nationalgeographic.com/2015/10/151020-city-sky-china-mirage-fata-morgana-weather-atmosphere/. Accessed 28 Dec. 2016.
Meyer, Edwin. "What Causes a Mirage?" Scientific American, 17 Nov. 2003, www.scientificamerican.com/article/what-causes-a-mirage/#. Accessed 28 Dec. 2016.
"What Is a Mirage?" Planet-Science.com, www.planet-science.com/categories/under-11s/our-world/2012/01/what-is-a-mirage.aspx. Accessed 28 Dec. 2016.