Sometimes, hot objects appear to "shimmer" due to the distortion of light caused by the movement of hot air molecules at different temperatures, creating variations in density in the air that disrupt light passing through these layers of moving air.
Everything around us is made up of molecules that are constantly moving due to heat. The hotter an object gets, the more its molecules become excited and move around like little hyperactive balls. This agitation is called thermal vibration. Imagine each molecule dancing in place: the higher the temperature rises, the faster and more chaotically it dances. Because of this intensive movement, the object becomes hot, and this agitation causes curious effects on the light passing around, creating the visual trembling sensation that we sometimes see above a fire or a road heated by the sun.
When the air heats up near a very hot object, its density drops, which changes its index of refraction. This simply means that light does not pass through the different layers of hot and cold air in the same way. As a result, the light rays are slightly deflected, bent, giving a blurry and wavy appearance to the observed objects. It almost looks like they are vibrating or trembling, simply because the layers of hot and cold air are constantly moving, slightly altering the direction of propagation of the light. It's a bit like looking at something through heated water: everything is muddled and undulates slightly.
When an object is very hot, the air around it heats up quickly, becomes less dense, and rises. The cooler, denser air takes its place, creating swirling patterns of varying density. This difference in density alters the path of light rays that pass through this area. As a result, we observe optical distortions, those famous ripples or waves of hot air visible above a road in summer or near a barbecue. These are fast and changing movements that slightly distort the image received by our eye, causing that characteristic blur and wavering effect.
When warm air rises above the ground, it creates a layer of air that is less dense than the surrounding air. This difference in density alters the path of light rays, bending them. As a result, our eye sometimes perceives distorted or even inverted images of distant landscapes; this is what is called a thermal mirage. It is typically this phenomenon that gives sun-heated roads a mirror effect, as we believe we see nonexistent puddles of water there.
The scientific term for this visual distortion is 'differential refraction.' It is used by physicists to indicate how differences in air density can bend the path of light and alter our visual perception.
The phenomenon known as 'thermal mirage,' like the one observed on the road in summer, does not only occur during periods of intense heat. This phenomenon can also take place in cold regions when the air near the ground is colder than the air higher up, thus creating surprising illusions!
In photography, disturbances caused by hot air can be used artistically. Some photographers intentionally capture these effects to visually convey the intense heat of a scene.
Professional and amateur astronomers often encounter difficulties due to atmospheric disturbances. To mitigate this effect, astronomical observatories are typically located at high altitudes or in regions with particularly stable climates.
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