For the uninitiated, a mirage is a mysterious trick of the air. Gaspard Monge (1746-1818), a mathematician who traveled with Napoleon’s army, was the first to discover the cause of this phenomenon.
He realized that the mirage was caused by a layer of hot air bordering the desert.
The difference in temperature and density of the air creates the phenomenon.
When the temperature difference is not large, it creates a haze, but when the temperature difference increases, it becomes a mirage.
heat shimmer
The most common atmospheric refraction we see around us is the heat shimmer.
They occur when sunlight hits hard. It’s a flow of air that starts near the ground and rises upward in a sparking motion.
You can see them on roads and sandy beaches that are easily heated. They often appear on sunny days in late spring or summer when the air is well heated.
Let’s take a look at the science behind how they form.
On a sunny day, roads are easily heated by sunlight. An asphalt road can reach temperatures of over 50 degrees when the daytime temperature is in the 30s.
The air at the surface is heated by the heated road.
Heated air becomes less dense, so it rises upward. It becomes lighter and more buoyant.
But while it may seem like all the air we see is the same temperature, it’s not.
The refractive index of light varies depending on the temperature of the air.
Therefore, light passing through a mass of air rising rapidly from the ground will be refracted differently. This is why landscapes appear to rise upward.
However, the temperature of the ground varies depending on what kind of ground it is, for example, if there is an asphalt road next to a field, the asphalt is hotter than the field, and there is also a difference in the temperature of the ground directly above the asphalt and much higher above the asphalt.
This creates multiple layers of air with different densities.
This is why you see a very small but continuous difference in density as the air rises. When light passes through those layers, it is refracted at different angles due to the density differences.
However, the temperature differences in the air are continuous, so there are no clear boundaries. That’s why we can’t see the light bending very well. Instead, it looks like the air is wobbling.
The temperature difference in the air isn’t large enough to cause the object to appear shimmering. You can see this phenomenon outdoors in early summer or in front of a fireplace in winter.
If the difference in air temperature is greater than this, a mirage is created. In denser, colder air, the speed of light is reduced, causing it to bend toward the cold side.
Conversely, in less dense, warmer air, the speed of light increases, causing it to bend toward the warmer side. This is why the actual position of an object is different from its apparent position.
mirage
A mirage is created when the temperature difference between the air in a cumulus cloud increases.
A mirage is a phenomenon in which an object appears to be at a location other than its actual position.
It is created by the refraction of light in an unstable atmosphere with large temperature differences in the air.
The best places to create them are deserts and polar oceans, where the temperature difference between the surface and the atmosphere is very large.
Deserts and polar regions have opposite air conditions.
Deserts have high temperatures at the surface, while polar regions have very low temperatures at the surface.
Therefore, mirages in both regions look different. There are two cases where mirages are created by hot air and cold air.
Mirages created by hot air are called “inferior mirages”. In summer afternoons, the ground temperature rises dramatically.
When driving on a heated asphalt road, you may see puddles of water on the pavement in front of you, and people and trees may appear to be growing.
The air temperature above the asphalt has risen significantly.
However, as you approach, the puddle soon disappears and another one appears some distance ahead. This is a type of mirage, caused by large temperature variations in the air near the surface.
This is what happens when the air on a road or desert ground on a summer day is more than 20°C warmer than the surrounding air.
Take a look at the illustration. In the desert, the temperature of the air at the ground level is much higher than at a height of 5 meters.
The light from the palm trees bends downward, initially into a region of cold air.
But there is hot air below. The light then bends back upward, toward the cooler air, and enters our eyes.
Because we perceive the light as coming from downward, the light from the palm tree appears to have traveled along an oblique path, as shown in the illustration.
In this case, the real palm tree is actually bending and entering our eyes upside down.
The brain mistakes this image for a distant palm tree on the horizon. Even though the light is refracted, the image is real and not an illusion, so we feel that the swaying image reflected in the sky is real.