What is a mirage?

Mirage is an optical phenomenon which creates an illusion of water. The most common occurrences are during hot sunny days and most of us are familiar with mirage we see on highways. We have heard popular stories of weary traveller who sees a lake at a distance, that’s just the reflection of the sky above which creates the illusion of blue lake.

So how is this illusion created in the first place? When light travels through a medium of equal temperature it follows a straight line path. But when there’s a temperature gradient i.e different layers of medium (air) have different temperature, light doesn’t follow a straight line path. This has to do with the refractive index of cold air and hot air. Refractive index is the ratio of velocity of light in vacuum to velocity of light in medium. So if a medium has higher refractive index the speed of light decreases in that medium. Hot air is less dense as compared to cold air so light travels faster in hot air than in cold air. During hot summer days the road or surface of earth gets heated a lot. This heats up the layers of air just above the surface and temperature gradient is created (regions of hot air above the surface). So the light coming from objects far away instead of following a straight line path towards us bend towards hot region as it travels faster through it creating a reflection like illusion.

Another explanation according to quantum electrodynamics is that the photons take the path of minimum time when travelling from one point to another. Even if the path is curve it will bend to reach other point in minimum time. So when a vertical temperature gradient is present during hot days mirages are formed.

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Types of Mirage

Inferior Mirage

Inferior mirage is when the image is formed under the real object. Usually in desert or highway mirage the real object is sky and the mirage is formed below the object which looks like reflection of sky from water. The light rays from object bent in hot region by same amount. Therefore inverted image is formed. This is the most common type of mirage. It is not much stable as the hot air rises above cold air which creates distortions in the image. As you walk towards mirages they seem to be moving away from you.

Superior Mirage

A superior mirage occurs when the temperature of air below the line of sight is colder than the air above it. This is unusual since warm air above cold air is unusual gradient and hence it is called temperature inversion. So now the light rays are bent downwards from the hot region, this creates the image above the object. This looks kinda weird and is not usually observed. They tend to be more stable than inferior mirages as there is no turbulent flow between cold and warm air. Superior mirages are common in polar regions especially over large sheets of ice that have a uniform low temperature.

These mirages can be pretty weird, some light from objects beyond horizon can bent and form an image above but the object cannot be seen as it is beyond horizon. This may explain some stories about flying ships or coastal cities in the sky, as described by some polar explorers. These are examples of so-called Arctic mirages, or hillingar in Icelandic.

Fata Morgana

Now this is something cool. Fata Morgana is an unusual type of superior mirage. A Fata Morgana may be described as a very complex superior mirage with more than three distorted erect and inverted images. Because of the constantly changing conditions of the atmosphere, a Fata Morgana may change in various ways within just a few seconds of time, including changing to become a straightforward superior mirage. The rays will bend and create arcs. An observer needs to be within an atmospheric duct to be able to see a Fata Morgana. Fata Morgana mirages may be observed from any altitude within the Earth’s atmosphere, including from mountaintops or airplanes.

This is a very good image of Fata Morgana. What is seen here is the city of Chicago from the town of New Buffalo, which are roughly 45 miles apart.