Definition of a Mirage
A mirage is an optical illusion caused by the bending or refraction of light. As it passes through layers of air of varying temperatures the deviation in the light path occurs. The deviation gets so strong that it results in Total Internal Reflection. Thus, the TIR makes the distort image of water-like appearance, or floating objects, confusing an individual beholding it. For a mirage to occur, the surrounding condition must be windless and very dry. Therefore, it is likely to occur in summer days in the deserts or on roads. They can also occur in cold dry regions and over oceans where the sunlight can quickly heat up the environment. At last, it is to be remembered that they are virtual parties, with no physical impact on the surface.
When light decides its path through the various layers of air it has to change its speed according to the air density at those portions. As a result, light reaches its destination, bending and curving and finally creating the illusion to a viewer of what is not actually present. Mirage is not always this simple. Various simple to mysterious images are found in nature whose understanding requires advanced knowledge of optics, light behavior and also of atmospheric physics. However, they can be well-understood by the principles of physics.
Atmospheric Refraction and Temperature Gradients
The refraction occurring due to the presence of varying atmospheric layers is known as atmospheric refraction. All the refraction and Total Internal Reflection credit goes to the density variation at different layers. The light as it moves through layers curves and sometimes the curving is so much that it exceeds the critical angle. This bending can also be caused due to the temperature and pressure variations of air at different levels. The same air can have varying refractive indices due to this reason. A temperature gradient comes into action which is the difference in temperature between two air layers. The differing temperature is usually between the ground level and higher altitudes. When the ground heats so much, such as on a sunny day, the air near the surface becomes at higher temperature than the air just above it.
This temperature difference also gives rise to a density gradient because hotter air is less dense and has a lower refractive index than colder air. Now, as a denser and a rarer medium are created, light of course bends away such that the gradient is greater enough. It causes light rays coming from the sky or distant objects to curve. Hence, it reaches an observer’s eyes from an unexpected direction. This curving light fools the observer, confusing the origin of the image showing and hence believing that something lies on the ground. Actually, the image formed is that of sky, which seems like a pool of water.
Conditions That Create Mirages Near Earth’s Surface
Mirages are formed under specific atmospheric conditions which give rise to temperature gradients which must be very strong. These conditions are easily achievable in the surroundings where the ground surface quickly warms up on the direct fall of sunlight. Deserts, paved roads, sandy beaches, and large rock surfaces are the most suitable places to obtain mirage because they absorb and radiate heat quickly.
A key condition is that the density gradient must be very high such that the refractive index of the upper layer is sufficiently greater than that of the ground layer. Light rays suffer this gradient and bend totally upward forming an inferior mirage.
Dry and wind free conditions stabilize the layers of air permanently in their place. This shows a better effect of TIR. When a layer of cold air sits below the hot air, a case of temperature inversion arises which can also produce mirages called superior ones, even in the colder regions. Thus, not only a phenomena of mirage formation but we can be familiar with what kind of mirage is formed, with the proper study of TIR.
Inferior Mirages: Desert Road and Hot Asphalt Effects
Inferior mirages are the mirages we are primarily familiar with. It occurs on the light rays travelling from colder air regions to hotter air regions. This type is usually seen in hotter environments like desert and hot roads giving an optical sense of water or a mirror. The inferior mirages are so called because the image appears below the actual object, such as the sky or clouds that display on the ground.
As light rays from the blue sky pass through the denser layers upward and enter the less dense, hotter air near the ground. They bend upward before interacting with the observer’s eyes. The image of sky reflects on the surface in front of us which gets misinterpreted by the brain and declares it to be water. The source of image cannot be identified by the brain and seems like the incoming light as having traveled in a straight line.
Although it looks realistic, the hot road and desert have no trace of water. This phenomenon is only an optical illusion. However, if the temperature, wind, or the observer’s movement changes, the illusion can immediately vanish. They are the interesting light phenomenon that occur naturally, also revealing the effect of temperature and pressure on light.
Superior Mirages: Objects Appearing Above the Horizon
Superior mirages are just opposite to the inferior images because the cold air gets near the surface and warmer goes above it. Thus, temperature inversion happens between the layers. They are specially formed over cold-water bodies and also frequently observed in polar or coastal areas.
Light coming from distant objects like ship, islands travels from the warmer air to the cooler, denser air close to the surface. This makes the incoming light bend downward which is the reason for the image formation, higher than the object. Superior mirages can cause the image of a real object to appear hovering in the sky, or several distorted images to gather above it.
Because the temperature-inversion can last long, these mirages can be more stable than the inferior mirages. They also spread on larger areas than the inferior ones. In the past, superior mirages have been frequently observed by Arctic explorers and sailors. These mirages have often tricked them for icebergs or landmasses that were not really present there. Superior mirages are more fascinating or amusing as they are located high above the horizon.
Fata Morgana: Complex Layered Superior Mirage
Fata Morgana is a complex form of superior mirage and is rare to obtain. It contains multiple layers of curved images piled up one another. It is believed that Fata Morgana was created by enchantress Morgan le Fay to lure sailors. They are formed under rare atmospheric conditions with complex temperature inversion, including very high refraction gradients.
In this illusion, light rays bend remarkably on passing through multiple layers of air varying on their temperatures. This gives the images of a single object that appear repeatedly distorted, elongated, inverted or duplicated. The mirage may be the resemblance of tall ships, islands, cliffs, or towers. They seem like floating in the sky or on the horizon, making it mesmerizing as well as mysterious.
For this to appear, it needs very large and flat surfaces like oceans or icy landscapes. It can be viewed generally during the early morning time or late evening as temperature differences become perfect at this time. Fata Morgana can shift rapidly, maybe within a few seconds.In the past decades, the rear view of these mirages gave rise to several tales and false observations, such as phantom ships and fictitious landmasses.
Arctic Mirages and the Novaya Zemlya Effect
The Arctic and Antarctic regions are in extreme temperatures so that temperature inversions are also extreme. This can produce exceptionally fictitious images called the Novaya Zemlya effect. This strong TIR occurs here when the sun rises earlier or sets later than expected. The resulting visible effect would be a flattened or multiple-image sun above the horizon. This was first discovered by the explorers in the Russian Arctic archipelago ‘Novaya and Zelmya’.
This phenomenon comes from bending of light as it suffers a curving from multiple atmospheric layers over hundreds of kilometers. A series of air layers with gradually decreasing temperatures and increasing densities are responsible for this bending. The rays of sunlight directly succeed these paths to reach on those surfaces of the Earth and hence the observer can see the sun when it is actually below the horizon.
Previously they were frequently used by the explorers to estimate the time and direction during the long and dark winter nights. This effect illustrates how strong is the refraction caused by nature, particularly in areas with extreme and stable temperature strata.
Looming, Towering, and Other Refraction Phenomena
Other related phenomena giving rise to a mirage are looming, towering, stooping, and sinking effects of the atmospheric layers. All are the outcomes of great temperature gradients.
- Looming makes an object that is normally hidden below the horizon visible, such as a ship appearing to float above water. This happens when light rays curve downward due to a strong temperature inversion, giving a superior mirage of the ship.
- Towering is the elongating vertical image of an object that makes it look taller than it actually is.
- Stooping means the compression of the actual size of the object that gives a shorter or flattened image.
- Sinking makes an object appear lower than its actual position due to the refraction of light through the air layers. These effects mostly occur together and can rapidly shift depending on the changes in the atmosphere of the layers.
These phenomena are the effect of refraction which come confusingly, especially at sea or in the polar regions.
Critical Refractive-Index Gradient Behind a Mirage
The formation of a mirage depends heavily on the refractive index of a strong gradient of the atmosphere. When temperature varies with the altitude the refractive index also changes accordingly. Sometimes, it may create a steep gradient within a short distance.
To produce a mirage this gradient must go beyond the critical level where light is able to escape out in the same altitude. Note that, the greater the temperature difference, the refractive-index gradient is stipend accordingly. Thus, the light curves more and more. Reaching the beyond the critical refractive index will trapping of light between the layers and lead to total internal reflection
Real-World Examples on Land, Sea, and Ice
Mirages are not a single environment property. They are diverse in surroundings and can spread in deserts, on hot roads, across oceans, and even over ice sheets. Here each environment has a unique satisfying atmosphere.
On land, the most familiar example is the inferior mirage seen on roads during hot summer days due to the hot air lying below and the cold one just above it.
Over the sea, superior mirages like Fata Morgana are formed because of the huge distant structures like ships, islands etc. This occurs due to the temperature inversion of the layers.
In polar regions, mirages like the Novaya Zemlya effect occur making the image of sun or distant landscapes despite being below the horizon.
These examples prove the diverse and global effect of mirage.
Mirage vs. Other Optical Illusions in the Atmosphere
There are several other optical illusions occurring on earth which are often compared with mirages. However, their distinction is required to properly understand mirage.
- Halos are formed around the sun or moon due to the refraction and reflection of light through ice crystals at clouds of high-altitudes. They appear circular and fixed around the light source.
- Sun dogs, or parhelia, are bright spots on either side of the sun, also caused by ice crystals. They remain stationary relative to the sun and do not involve refraction due to the temperature gradients as like mirages.
- Glories are rings of light seen around a person’s shadow on a cloud or fog bank. This is caused by diffraction.
- Green flashes occur shortly during sunrise or sunset when atmospheric refraction disperses the sunlight.
Above cases do not require strong temperature gradients or even less amount of temperature gradients to occur. Thus they are totally different from mirages.
Misconceptions and Fun Facts About Mirages
Mirages are greatly misunderstood, giving rise to myths and misconceptions. Some of them are as follows:
- People believe that mirages are hallucinations, especially in deserts where thirsty travelers are supposed to see sources of water. In reality, they are not the hallucination created by the mind but the tricks of light which are physical phenomena.
- Mirages occur in all climates ranging from deserts to arctic and antarctic poles and not only confined to hot climates.
- Fata Morgana is taken as a mysterious scary tale. However, it is just a mirage.
- Another interesting fact is that mirages can sometimes be captured with cameras, especially powerful zoom lenses.
Despite their vague and complex reputation, mirages are not magic but 100% a physics work. It might be virtual that doesn’t exist but based on real physics principle.
Practical Implications for Navigation and Safety
Although mirages are beautiful and fascinating, they can also have serious consequences due to the mind games. For sailors, pilots, and desert travelers, mirages may lead to dangerous misinterpretations of the environment causing physical damage.
- At sea, a superior mirage might cause a ship to appear closer or larger than it actually is. This confuses radar readings or visual navigation.
- In desert regions, inferior mirages can make travelers believe that water lies ahead. This can make them wonder without any reason and waste time and resources.
- In aviation, mirages near the runway can affect a pilot’s perception of distance and altitude during landing. Although modern instruments can lower this risk, pilots are still trained to recognize and account for visual illusions.
- On highways, road mirages may give a false impression of a wet or slippery surface, to the drivers. This can hamper the reaction time.
- For scientists and meteorologists, understanding mirages can provide valuable insight into atmospheric conditions and monitoring the environment.
In all cases recognizing a mirage and understanding the science behind it is crucial.
Conclusion
Mirages are mesmerizing environmental tricks. They can potentially be beautiful as well as harmful if not sensed properly. They can appear in challenging environments of summer to freezing cold environments. Oceans and sailors are the frequent victims of mirages. The process of mirage formation may extend from simple TIR to complex TIR like the Novaya Zelmya effect.
All the types of mirage formed are something that do not exist but create a serious impact on an observer. However, they are not hallucinations but are grounded by physics. One can get mirage confused with other optical natural illusions. So, proper knowledge of critical angle, Total Internal Reflection and temperature gradients is required to potentially recognise a mirage.
References
Richey, L., Stewart, B., & Peatross, J. (2006). Creating and analyzing a mirage. The Physics Teacher, 44(7), 460-464.
Fraser, A. B., & Mach, W. H. (1976). Mirages. Scientific American, 234(1), 102-111.
Jha, A. S. (2020). Mirage as the Reflections of the Differences across the Boundaries: Some Observations for Insight.
https://en.wikipedia.org/wiki/Mirage
https://byjus.com/physics/mirage/
https://www.britannica.com/topic/mirage-optical-illusion
https://www.worldatlas.com/articles/what-causes-a-mirage.html
