Refraction causes wavy lines above hot surfaces due to the bending of light waves passing through cold and warm air with different refractive indices. This phenomenon, responsible for desert mirages, occurs due to the unstable air temperature causing vibrations. It is important to drive carefully during “highway mirages”. Inferior and superior mirages occur when the image appears smaller or above the real object, respectively.
The wavy lines that appear above a hot grid or asphalt road in midsummer are caused by a phenomenon called refraction. Refraction is the bending of light waves and occurs when light passes between substances with different refractive indices, in this case cold air and warm air. Because hot air is less dense than cold air, light accelerates as it reaches a hot surface and then curves back up, causing the viewer to see an image of the sky and the surface to appear wavy in the heat . This phenomenon is responsible for desert mirages, as refraction and reflection look very similar to the human eye, and the refracted image of the sky from very hot sand can look like the reflection of the sky in a pool of water.
The reason things look wavy in the heat, instead of looking like a stable picture of the sky, is that the air temperature isn’t stable. Hot air rises naturally, so as the air escapes the hot surface, it rises for a while before rapidly cooling and sinking to be reheated. This constant mixing of hot and cold air produces vibrations, resulting in refracted images that have a wavy appearance. A road that looks wavy in the heat can be dangerous, as it can look like water or oil has been spilled on the road, so it’s important to drive carefully when it’s hot enough for so-called “highway mirages” to appear. The sands of a desert are also prone to mirages, which thirsty travelers can famously interpret as an oasis.
When things appear wavy in heat, the phenomenon is known as an inferior mirage, because the image seen is smaller or smaller than the actual object, in this case the sky. A superior mirage is also possible, in which an image appears above the real object. For this type of mirage to occur, warm air must be higher in the atmosphere than cold air. An example of a superior mirage is sunlight which is sometimes seen above the horizon when the sun itself is below the horizon.
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