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When driving on a wet highway at night, any surface emitting or reflecting light will have a reflection right under it that is blurry and seems to stretch to infinity.

This is not exactly what I am picturing in my mind, but I couldn't find better images. Anyway, I think it'll do.

Does anybody know why it is such a long reflection? I never figured it out myself.

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marked as duplicate by John Rennie, ACuriousMind, Community May 21 '15 at 14:38

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Light from the source is emitted in all directions - including towards every point on the road surface.

When the light hits the (wet, rough) surface of the road, it is scattered in a variety of angles. Some of the light rays will be scattered at just the right angle to enter your eye.

Therefore reflected light from all different parts of the road between you and the light source enter your eye - brightest at the points at the "ideal" reflection point - so you see a blurry stretched image of the source.

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    $\begingroup$ And the scattering in all directions is mainly due to the roughness of the road. $\endgroup$ – Sebastian Riese May 20 '15 at 20:55
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    $\begingroup$ I have considered this possibility, but it puzzled me that the reflection is always exactly vertical. With scattering plus roughness, shouldn't we see some of the light from different trajectories (i.e stretching a bit sideways) too? $\endgroup$ – André Chalella May 21 '15 at 0:36
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    $\begingroup$ Actually this "sideways scattering" does happen. It can be viewed more clearly in the sun reflection in the linked question (duplicate). I also figured my other doubt, which is "why is it vertical?": it's just the way the brain interprets the virtual image, I guess; it could interpret the image as laying on the road towards you as well, right? $\endgroup$ – André Chalella May 21 '15 at 15:41

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