What is Retroreflectivity?

Next time you are travelling in a car at night think about how the signs you see on the roadway magically light up and can be read by you in the car as you pass.

These signs are visible to you through the phenomena called retro-reflection. As your car gets close to the sign, the car headlights shine onto the sign surface. The sign surface has little reflective elements, either glass spheres or triangular prisms (pyramids) that take the car headlight beam and bend it back or 'return' it to the car driver and passengers. Hence you can see and read the sign.

As this image shows, the retroreflected light returns to the car driver as a cone of light. Anyone outside this cone of retroreflected light will not see the sign at night.