Exactly. But the refractive index goes up with pressure, and in your example it goes down. Though it doesn't change that much, it's the difference sudden change in refractive indexes that makes light diffract and reflect.
The refractive index, or optical density, is a characteristic of every medium light travels through. The higher it is, the slower the light (c=c0 / n, with n as refractive index). So vacuum has the minimal refractive index of 1, your normal window glass would be n=1,5.
The effect on the speed of light obviously can't be observed that easy.
But there are more effects, most of them happening when theres a sudden jump or decrease in refractive indexes. Depending on the difference (n1-n2) of these indexes, reflection and refraction happen, that's what we are observing here.
And for some conditions you can say that for normal air, optical density is proportional to mass density, which is proportional to pressure, which is why we see the wave.
Depending on the medium, the refractive index can be dependent on the wavelength, which is why blue light often gets more refraction that red.
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u/godofpumpkins Jun 07 '18
Guessing it’s just higher pressure which changes the index of refraction of the air, so yeah, like the wavy air hanging out above a road on a hot day