I noticed the same type of thing happens with fire.
In: 1
[Refraction](https://en.wikipedia.org/wiki/Refraction) is when light changes direction when going from one (transparent) material to another. You can read the wiki article to find out exactly why; it has to do with light acting as a wave, and the wave moving differently inside each of the two materials.
Gasoline fumes are transparent but they have a different *density* than air. And HOT air (such as right next to the flame of a fire) also has a different density than the colder air between you and the fire.
So all in all, different density = refraction happens. You see it as that shimmering. The [shockwave of an explosion](http://newsimg.bbc.co.uk/media/images/47480000/jpg/_47480955_explosion1_copy.jpg) is also very different density air, and also causes refraction like that.
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