Why Do We See Mirages?
– A Trick of Light Played by Air Temperature Differences
On a blazing summer day, have you ever looked down a long asphalt road and seen a shimmering light, as if there were a puddle ahead? When you walk toward it, the "puddle" never gets any closer. It seems to keep running away, always about the same distance ahead. This is called a mirage. So what is going on in the air?
On a hot day, on a road or in a wide open place like a desert, the scenery near the ground can seem to wobble, or a band of light like a water surface may appear far away. This band of light is not really water. It is said to be light from the sky "reflected in the ground".
Why do we see a reflection like water where there is nothing?
When strong sunshine heats the ground, the air just above it heats up too, and becomes lighter (less dense) than the air around it.
When density changes smoothly from place to place, light is said to travel while turning a little at a time (refracting).
Let's look at how this "density differences bend light" idea works, step by step.
Near hot ground, a layer of lighter air forms
Ground heated by strong sunshine warms the air just above it. Warmed air expands, and its weight per unit volume (density) becomes lower than that of the surrounding air. In this way, a thin layer of air lighter than its surroundings forms right next to the ground.
In air of varying density, light gradually changes direction
When light crosses the boundary between materials (or air) of different density, it has the property of being bent slightly. In a layer of air where density changes continuously, as near the ground, light is thought not to bend sharply all at once but to turn its direction a little at a time, smoothly. Light from the distant sky, coming in at a slant toward the ground, is bent gradually upward as it nears this hot layer of air. As a result, to the observer's eye, it looks as if it has arrived from the ground.
It is simply real light from the sky, sent in the wrong direction by the lens of a temperature difference in the air.
Why does a mirage always look like "water"?
Our brains are said to readily interpret bright light, such as light from the sky, arriving from near a flat surface like the ground or a road as a "reflection on water". When the sky is reflected in a real puddle, light of the sky's colour likewise reaches the eye from near a flat surface. The light seen in a mirage arrives in a way similar to this "reflection on water", so the brain is thought to mistake it for a puddle.
Unlike the mirage caused by hot air near the ground, light can also be bent into a different kind of mirage when the air near the sea surface is colder and heavier than the air above it. In this case, distant scenery can appear higher than its actual position, and ships or islands sometimes look as if they are floating in the air.
What you can check for yourself
- On a clear summer day, look out over a hot asphalt road or car park from a safe place at a distance
- Watch whether the scenery near the ground seems to wobble
- Look for a shining band like a puddle below distant cars or scenery
- Look at the same place at a cooler time of day, or from the shade, and compare the shimmer
Watch out for heatstroke, and observe from a safe place such as the shade or inside a car.
Summary
A mirage is said to be caused by a layer of air near hot ground that is lighter than its surroundings. As light travels through air whose density varies from place to place, light from the distant sky is bent little by little on its way, and we see what looks like a reflection on water. It is also thought that our brains readily interpret light from near a flat surface as a "reflection", which is part of why we mistake it for a puddle.
The "puddle" you see down the road is not water. It is an illusion in the path of light, created by the air itself.
The same property, where "a wave's path bends where its speed changes gradually", also helps whale songs carry far through the sea. You can read more in this article. For an example where the shape of a lens, not air, bends light more sharply to a single point, see the magnifying glass article.
For those who want to know more – terms, numbers and links to textbooksFrom middle-school science to topics under research, each part states which level it belongs to
- Middle schoolCovered in middle-school science
- High schoolCovered in high-school "Basic Chemistry" and "Basic Physics"
- High school+Advanced or sidebar content in high-school textbooks
- UniversityUniversity-level content (atmospheric optics) not taught in high school
- ResearchTopics researchers are still investigating, not taught even at university as settled knowledge
Middle schoolTerms: words around mirages
- Refraction: the change in the direction light travels at the boundary between materials with different properties.
- Density: weight per unit volume.
- Inferior mirage: a mirage in which the scenery appears reflected below, caused by hot air near the ground.
High schoolChecking with a formula: finding the density ratio from temperature
Here we work out what we can. Air temperature and density are linked by a well-known relation: "at constant pressure, the volume of a gas is inversely proportional to its absolute temperature." We use this to calculate a density ratio.
Density ratio = lower absolute temperature (K) ÷ higher absolute temperature (K)
| Absolute temperature | Celsius temperature plus 273 (unit: K, kelvin) |
| Density ratio | 303 ÷ 323 ≒ 0.94 |
| Result | The hot air near the ground has about 94% of the density of the air a little above it (about 6% lighter) |
| Density ratio | 298 ÷ 333 ≒ 0.90 |
| Convert to a percentage | 0.90 × 100 ＝ 90 |
| Result | The hot air near the ground has about 90% of the density of the air above it (about 10% lighter) |
We can see that the bigger the temperature difference, the bigger the density difference. This density difference is thought to be linked to how strongly light is refracted.
High school+Why we can't take this further with a formula
So far, we can work out the density difference of the air from a reliable relation. But to calculate accurately how much, and along what path, this density difference actually bends light, we would need a more detailed optical relation linking density to how easily light refracts (the refractive index), and data on the complex temperature profile, which changes moment by moment with height above the ground. Wind strength, terrain and the angle of the sunlight are also said to change this temperature profile a great deal. This article stops at what can be said with confidence, that "a density difference forms", and does not calculate the exact bending.
UniversityThe complex paths of refraction in atmospheric optics
In atmospheric optics, researchers use computers to simulate the path light takes, based on data for how temperature changes with height. It is said that stronger mirages are more likely to be observed where the temperature changes more steeply near the ground.
ResearchWhat is still unclear
- Techniques for predicting and measuring more accurately the temperature profile near the ground, which changes moment by moment, are still being studied in atmospheric science.
- The detailed conditions that produce complex mirages in which ships or islands appear displaced from their real position are also still being investigated from the viewpoint of meteorological optics.
- Research is also under way on techniques that observe the state of the atmosphere optically, applying the same principle of refraction as mirages.
Even a fleeting sight far down a road holds a rich topic, where atmospheric science and optics meet and research continues.
Links to the textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Middle school | Science: refraction of light | Basic terms: refraction, density, inferior mirage |
| High school | Basic Chemistry: properties of gases | Absolute temperature and density ratio calculation |
| High school+ | Physics: refraction of light (advanced) | Why we can't go further with a formula |
| University | Atmospheric optics | Light-path simulation from temperature profile data |
| Research | Atmospheric science, meteorological optics (under research) | Predicting temperature profiles, conditions for complex mirages |
- Explanations of the relation between refraction of light and density in physics textbooks.
- Explanations of the relation between gas volume and absolute temperature (Charles's law) in chemistry textbooks.
- A research review of the mechanism of mirage formation in atmospheric optics.
- Explanations of near-ground air temperature distribution and refraction of light in meteorology materials.
- Explanations of the perception of reflected light in visual psychology materials.
* The temperature and density values are example figures to help explain the mechanism. Working out the real relation between density and refraction of light needs more detailed optical data, and this article does not calculate exact refraction angles.
* This article is a general-audience science explainer. When observing outdoors in hot weather, take great care to avoid heatstroke, and rest in the shade or a cool place.