How Are Mirages Formed?

We all have seen them. We all have pointed them out to our parents as kids. We all have been told that they are called mirages. And we all have accepted that answer without ever trying to understand the fundamental but amazing forces of nature at work in this simple occurrence. 

► Mirages form due to refraction of light. Due to its nature as a wave, light's path changes when its medium changes. The amount of the shift in direction is determined by the refractive index of the medium it enters. Contrary to popular misconception, light does not actually 'bend' when shifting mediums, but merely changes its direction to a different, but straight, path. The 'bending' effect in mirages is simply the cumulative result of multiple successive refractions. 

► Mirages are caused due to the difference in temperature of the numerous layers of air above the ground, and are seen on extremely hot days. The temperature of the air above the ground is determined by the temperature of the ground itself, since solar energy does not have a significant effect on the temperature of the air itself. As solar energy heats the ground, the air closest to it gets warm, and the layers above it become progressively cooler. On normal days, the difference in heat is not enough to cause a sizable deviation in the path of the light. However, on extremely sunny days, the air closest to the ground becomes extremely hot. Air gets rarer as its temperature increases, and its refractive index decreases as it does so. 

Refraction Of Light

► Light reflecting off an object (say, a tree) undergoes a change in mediums due to the different temperatures in the air. As explained in the adjoining illustration, light bends away from the normal when entering a rarer medium from a denser medium. As it travels towards the viewer, the light is first bent away from the normal several times, each refraction increasing the angle at which it enters the next medium. This cycle continues until it enters a hot layer of air on an almost parallel trajectory, and is refracted upwards. This is known as total internal reflection (TIR). 

► After being eventually refracted upward by one of the hottest layers of air, the light keeps on bending upwards (without TIR) as it encounters cooler layers of air. As seen in the illustration, when entering a denser medium, light bends towards the normal. Thus, instead of following a straight path, the light bends upwards and ultimately reaches the viewer. 

► Our eyes and brain, fascinating though they may be, can't work out that the light has followed a curved trajectory. The brain simply accepts the direction of the light received by it as the one the light had been traveling the whole time, and 'sees' the object by tracing back the path of the light it has received. However, as we saw, the light that reaches the viewer is actually traveling in a tangential direction to the real, curved direction of the light. So, the brain 'sees' the object at a lower height than it actually is. Because the brain perceives the object to be at a lower height than it actually is, this type of mirage is called an inferior mirage. 

► Superior mirages are seen when the surface is very cold. As you may have guessed, superior mirages are mirages where the object appears to be at a greater height than it actually is. These mirages are usually observed over cold seas, since water takes much longer to warm up than sand or rocks. 

► The principle behind superior mirages is exactly the same as inferior mirages, except the layers of air are inverted. When water is very cold, the air adjacent to it also remains cold, with the temperature increasing along with the height. Since the lower layers are denser than the upper layers, a ray of light reflecting off, for instance, an iceberg will first be bent upwards to the point of TIR, and then bent downwards towards the viewer. The brain accepts the last direction as the one maintained by the light the whole time, and perceives the object to be higher than it really is. 

► In both superior and inferior mirages, the relative positions of the object and the viewer are crucial. The mirage can only be seen in certain zones around the object, and the viewer will not be able to see it when he moves outside those zones. 

► In contrast to optical illusions or hallucinations, mirages are real images, since light actually enters the viewer's eyes. That it is interpreted incorrectly by our brain doesn't make the phenomenon any less real. Mirages can be photographed. On the other hand, hallucinations are caused by chemical imbalances within the brain, which create false images without any optical input, or psychosis. 

The apparently miraculous sight of a mirage is thus, nothing but our brain becoming befuddled! And we thought our brain was the greatest natural marvel in the world!