One of the roads near my house was just redone. They added some awesome retroreflectors in the middle. Here is a shot. No wait, I don’t have a picture of that. I tried to take one, but it just didn’t turn out very well. Oh, you know what it is supposed to look like. It looks like little tiny lights in the middle of the road.
What makes these things so cool? Why do they look like they are battery powered or something? Maybe it would be helpful to compare retroreflectors to some other materials. I can group stuff in the following manner:
- Shiny stuff
- Non-shiny stuff
What if I place some of these things on the floor and view them in different arrangements of viewing location and light source? Here are two pictures. For the first one, I took a picture of the objects with a flashlight very near the camera. For the second picture, the flashlight is not near the camera.
- First, I messed up. I had to set the camera on a long exposure to get these to come out ok. For the first one, I bumped the camera and made the image look all fuzzy. Didn’t even realize this was messed up until I picked up all my stuff.
- I put some stuff in there. I have the retroreflector on a pair of running shoes, and a retroreflector from a bike. Also, I put a mirror in there as my “shiny” object. The ground acts as a non-shiny object.
- For the top image, the light is right near the camera. Notice how shiny the two retroreflectors look.
- For the bottom shot, the two retroreflectors look non-shiny and everything else looks the same as it did.
- In both shots, the mirror looks black.
So, why is it this way? First, the shiny object. For this, light bounces off in the same way it hit the object. You don’t see anything when you are holding the flashlight because the light bounces away from your eye. Here is a light ray diagram for that case. I put an eye on the other side so you would be able to see the light from the flashlight.
Here is a diagram for a non-shiny object. When light hits it, it reflects in many different directions. Some of the reflected light goes back to the eye, so you can see it.
I made the arrows representing the reflected light smaller to represent the idea that each of these is not as bright as the original. And now, here is a diagram for the retroreflector.
When light hits the retroreflector, it comes back the same way it came. This doesn’t really depend on what angle the light is incident on the material. Note that a normal mirror will also reflect light back the same way it came – but one when the light hits perpendicular to the mirror.
How do you make a retroreflector? One way is with perpendicular mirrors. Here is an example in 2-dimensions:
And if you built one in 3-D, it would look like this:
If you have three mirrors, you can easily reproduce that configuration. The cool thing is that if you hold a flashlight near your eye, no matter where you go in a room it will shine back on you. Try it.
Of course, there is another way to make a retroreflector – tiny little glass spheres. These will also reflect light back in the same way it came in. Look closely at a retroreflector and sometimes you can see these.