Take a look at all these photos of city streets in the rain. I found them all on google image search and I am shamelessly hotlinking them:
I was driving home from work today in the rain, and saw many similar scenes. However I noticed something interesting that I have never noticed before. The street lights, car lights, traffic lights, etc. all reflect off of the wet surface of the road. However since the road is not a mirror-flat surface, the reflection is smeared out or diffused due to the rough texture of the road. So my question is this: why is the smearing or diffusion of the reflection almost entirely in the vertical direction? Certainly the surface of the asphalt or concrete does not have a sufficiently heterogeneous asperity (i.e. the roughness is uniform from all directions) to account for this. Shouldn’t the light spread out as far horizontally as it does vertically? Any insights here?
I’d guess it must be a function of the water as it doesn’t do that in dry conditions. Is the light focusing the light a la moonlight on still water?
Well, the water on the surface of the road makes it reflective (just like a water puddle or a lake, just thinner).
You are right though, the same thing does happen with the moon on a the surface of a lake. See here
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