How Do Retroreflectors Give You Super Vision at Night?

It was a dark and stormy night.

(I've always wanted to start off with that line.)

You were driving home from work. The rain had finally stopped, but the roads were wet and shiny, making it difficult to see the lane markers painted on the surface. It seemed like the headlights only made things worse. But after turning onto another road, everything got better. You could clearly see the lines on the street, as though battery-powered lights were illuminating your way. But that light was actually coming from slightly raised markers called retroreflectors. These are optical devices that always reflect light back to the source from which it came—no matter which orientation the device has.

How do these help you navigate at night? Let's start with some basics.

How Do You See?

Whenever you look around, you can see two types of objects. The first produces its own light, like an incandescent bulb, a candle flame, or the screen on your television. The other type doesn’t. Instead, light reflects off the object. Imagine a pencil on your desk. If the desk lamp is on, you can see the pencil, because it’s reflecting light from the lamp. If the lamp’s off, you can’t see it.

But whether or not an object is shiny also affects how well you can see it in low-light conditions. When a beam of light hits a super-shiny object like a mirror, it reflects off it in a very particular way.

Here are two lasers (a green and a red) aimed at a flat mirror. I’m using two different colors to show two different paths of light. There’s nothing particular special about these two colors, except that they look cool.

Photograph: Rhett Allain

For both of them, their light hits the mirror at the same angle that their light also reflects from the mirror—but in the opposite direction. (Both angles are measured from a line perpendicular to the mirror.) Physicists say that "the angle of incidence is equal to the angle of reflection." Here is a diagram that should help you visualize this:

Illustration: Rhett Allain