Optics, rain, and car headlights

One of the fun things about being a physicist is that occasionally you end up pondering some sort of everyday phenomenon that you’ve never thought about before and realize that you can explain it with some elementary physics! Some time ago I had one of these little epiphanies and thought I would share it.

Have you ever been out driving at night after a rain storm and found yourself constantly checking to see if your headlights are on, because they don’t look like they’re on? We can explain this very simply with a basic discussion of optical reflection.

First, we can ask: how to you normally notice that your headlights are on at night? If you’re right behind another car, or passing by a nearby road sign or other roadside feature, you can probably see the light from your headlights reflected back at you. But you can also see the reflection of your headlights from the road ahead of you.

In fact, because the pavement is a rough surface, the light from the headlights scatter in all directions when they collide with the road. This is what we call diffuse reflection, and it is the most common type of reflection you see. Most surfaces are not terribly smooth, and so the light hitting it scatters every which way. In the case of the car, some of the light from the headlights bounces back towards you, so you can see, indirectly, that your headlights are on.

But what about when it has rained? If there is enough water on the pavement, it fills in all the little crevices in the pavement, making it effectively a smooth surface, like a mirror. Then you get the elementary case of what is referred to as specular reflection, where the angle of reflection \theta_r is equal to the angle of incidence \theta_i. This means that nearly all the light from the headlights ends up being directed forward, away from the car, and you don’t see any light on the pavement ahead of you.

Since we’re so used to being able to see some light reflected back on dry nights, we might doublecheck to see if our headlights are on!

Of course, if it is actively raining while you’re driving, you’ll often see light scattered off the raindrops themselves, which is why I have stressed that this is an effect that you see soon after it has rained.

Pretty sure this explanation makes sense, though I’ve never seen anyone talk about it before! It’s a nice little example of how a little bit of physics knowledge can often illuminate things you see in daily life (no pun intended).

Postscript: This also gives you an idea of why stealth aircraft are designed to have flat surfaces! Ordinary aircraft, with round bodies, tend to reflect radar waves in all directions, including back to radar defense systems, making them easy to track. Stealth aircraft, with their mostly flat surfaces, tend to reflect radar waves only in the specular direction, which is very unlikely to be in the location of a radar detector!

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1 Response to Optics, rain, and car headlights

  1. Phil Buckley says:

    I’m not a physicist but an electronic design engineer who grew up with a mechanical design engineer father who dabbled in optics as part of his job and, as a result, was introduced to prisms and lenses almost before I could walk! Your precise explanation above goes through my head EVERY time I’m driving on dark wet roads and, just this week, found myself explaining the effect to my long suffering wife for probably the Nth time who accepts that the roads seem ‘dark’ but isn’t interested why. My daughter is a physicist and a while back told me she’d been watching a fairground ride and found herself analysing how it worked in her head – she asked me if this is how life was going to be for her!? I answered ‘yes’ and felt an immense sense of pride.

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