Light Reflection in the Rain

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:

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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?

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2 Responses to Light Reflection in the Rain

  1. spencer says:

    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?

  2. Derek says:

    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 heremoonlight.

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