Title says it all basically.
Thanks for the amazing answers
In: Earth Science
Light always takes the shortest path _in time_ (how it knows which path is the shortest in time: at the level of quantum fields it takes _all_ paths and the one that is the fastest is realised). Also the speed of light depends on the medium it travels.
Now, hot air is less dense than cold air, resulting to less refraction (photon collides less often), so e.g in deserts you are literally seeing the light coming from the sky bent so that it seems like there is water (its just a distorted image of skies) in the sand.
All this because it is faster for light to bend and move through the hot air rather than take the direct straight path between the sky and your eyes.
This is physics under the subject of quantum electro dynamics and there is a fine popularised book (containing essential the same explanation) by Richard Feynman called “QED: the strange theory of light and matter” which I highly recommend. I’m a 3rd year PhD student in particle physics and cosmology.
Ground hot; air less so. Ground heats air. Air must rise, cooler air must fall to take its place. This causes chaotic motion between the hot and cold air, forming moving streams of the two. Hot air allows light to travel faster than cold air, and so the light bends when passing between the two, forming a shimmering effect which can be vaguely similar to what light does when it goes into a pool and gets refracted by the waves.
Refraction varies with temperature. Air close to the ground will be hot and at a lower density than air just above it. These differences lead to differences in the speed of light so light bends as it moves through these layers. Light from the sky bends back up toward your eyes so parts of the ground look blue.
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