The apparent wavering or flickering of objects is caused by atmospheric refraction in a local environment. The air just above a heat source becomes hotter, making it lighter (less dense) than the cooler air above. Since the physical conditions of the refracting medium (air) are not stationary, the refractive index fluctuates, leading to the apparent fluctuation in the position of the observed object.
What causes the apparent wavering or flickering of objects observed through a turbulent stream of hot air rising above a fire or a radiator?
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The apparent wavering or flickering of objects seen through a turbulent stream of hot air, known as a heat haze, is caused by the variation in air density. As the air near a hot surface becomes heated, it rises and creates pockets of different temperatures and densities. Light passing through these fluctuating air layers experiences refraction, causing the visual distortion. This bending of light at different temperatures leads to the shimmering effect observed. The phenomenon is similar to the distortions seen above a hot pavement or a grill, where the temperature gradients in the air result in the bending of light waves and the perception of a wavy appearance.