Everyday Physics: Driving in the Evening When It Is Wet

I had to go out to the store tonight to pick up milk and few other basics. It had been drizzling much of the day so the roads were wet, but it was not raining so much that visibility was affected. However, it was harder than normal to see. It seemed darker than it should. If I lived out in the country, then I might have blamed the clouds for blocking the moonlight, but I live in a suburban area where there is plenty of artificial light, streetlights, car headlights, lighted homes and shops. Why does it seem so dark?

It is the wet streets, so that light from my headlights reflects specularly. Specular reflection is when a very smooth surface causes all parallel rays of light to be reflected the same way, and in the this case it is away from me. Click the link above to see a diagram. On a dry night the black road surface absorbs lots of light, but it also reflects some diffusely or in all directions. Some of that diffusely reflected light comes back to me, some goes sideways and helps to illuminate things around the road, and some just goes away from me, On a wet night most of the light from my headlights reflects away from me, Even the overhead lights lose some of the usefulness. More of the light gets reflected back up at the light, rather than toward me where it would help me see.

If that wasn't bad enough the general lack of illumination is complemented by extra glare. All of that light from my headlights that it is not coming back at me is being directed into the eyes of drivers coming the other way.

I liked to use this story when I taught freshman optics, but it would never rain the night before, and most freshman do not drive much. It does help much, if the class is not familiar with the experience you want to relate to the lesson.

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3 Responses to “Everyday Physics: Driving in the Evening When It Is Wet”

  1. One way you could have brought the demonstration into the classroom without having to rely upon the weather would be to paint a piece of board with black oil paints. Then, when it’s dry, brush varnish onto half of it. That is a dramatic demonstration of how the apparent darkness of the paint depends upon the finish you put on it. Black surfaces appear only a fairly dark gray when they dry to their natural, specularly reflective matte finish–you have to put on a glossy varnish to bring out its full luster.

  2. In addition to the point about how reflection works, I also wanted to make the point about applying what was learned in class to the outside world. There is physics out there if you look for it.

  3. Absolutely! I loved learning about Newton’s rings my freshman year…I’d never known why slides stuck together appeared to have those banded patterns.

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