Specular reflection and driving at night

One night I was driving on a winding road near Golden, CO and wasn’t sure if I was in my lane or not. There were four lanes so I know I was on the right side of the road. But I could not see any markings between the two lanes going in my direction and the road was really dark for some reason. I’ve thought about that situation enough to try and figure out why that happened. It’s not just that road I was on but something on the road.

Do you know how our headlights work? Silly question. Those beams shine out along the road in front of us and the road surface reflects the light back into our eyes. That’s a particular type of reflection called diffuse reflection. You are involved in the process whether you knew what it was called or not. If light didn’t bounce back into our eyes we wouldn’t see the road. Not a spectacular deduction but that’s how we see where we’re going. And thank goodness the surface has these little “imperfections” that do that. But what if those little imperfections, those little teeny bumpy surfaces in the road, were filled in with water, maybe just a little bit like after a sprinkle of rain or from melting snow running across the road?

When moisture fills up all those little imperfections in the road this is why the road is harder to see. The light from our headlights hits less of those imperfect reflecting surfaces, we get less diffuse reflection into our eyes and it just bounces up further down the road (away from us). That’s called specular reflection. Light is bouncing off the surface of the water and heading in the same direction as our car (truck, whatever). It ain’t coming back to help us see the road like when it was dry. What are we going to do about that? Not a whole lot unfortunately.

I can make a couple of suggestions though. Clean your windshield, clean the gunk off your headlights and slow down. Don’t drive? Get those glasses out of the glove box and put them on 🙂

An example of a beneficial type of specular reflection is when photographers use the smooth surface off of water that is reflecting mountains or some landscape. That’s kinda of like a reverse specular reflection.

Here’s a little more information from this website that might help understand the optics:


wet road

wet road