Asked by: Sarah Shepherd, by email
It's for the same reason that a straight stick appears bent when some of it is underwater - because the light is passing between media that have different refractive indices (the ratio of the speed of light in a vacuum to that in the medium), which means that light rays are 'bent' as they pass across the interface between the two. The bigger the difference in refractive indices, the more the light rays are bent. The effect is quite large when light passes between air (refractive index 1.0003) and water (refractive index 1.33). It is much smaller when the light passes between air and a mass of petrol vapour (refractive index 1.0017), but the difference is still enough to deflect light rays so that the boundary between the vapour and the air becomes visible as a shadow on the ground. If you look closely, it's even possible to detect variations in concentration within the mass of vapour as fluctuations in the intensity of the shadow.