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
  • Retroreflectors

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.


Some notes.

  • 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.

More like this

i think this is the same sort of retroreflective device left on the moon. you can shine a laser at it and get the time of flight to the moon and deduce it's distance.

I'm a big fan of retroreflectors. On the photography side, flash photographs of them become more dramatic as you get further away from them.

An unrelated fun fact - astronauts left a panel of retroreflectors on the moon, which are used for laser ranging of the earth-moon distance.

As for what's inside retroreflectors in the road: you wouldn't want corner cube retroreflectors beacuse they're too good! Most of the light from a car's headlights would be reflected back to the headlights instead of the driver's eyes.

As you point out, the road things use spheres. But aren't retroreflectors in quite the same way as the corner cube. What goes on in the sphere is the same effect that gives rise to rainbows: there's a "magic angle" for the combination of refraction and reflection in the sphere that concentrates a large fraction of the incident light winds up in a narrow angle. (For the rainbow, dispersion in the water makes that magic angle slightly different for the different colors, of course.)

If you have an undergraduate optics class where you explain the rainbow and calculate the "magic angle" for a sphere of water, a fun problem is to reuse that calculation to determine the index of refraction required to the magic angle to occur at retroreflection. It's quite high (around 2 if memory serves me correctly), so I have no idea what kind of material they actually use in to make the retroreflective panels in the road and in my running shoes.

By Anonymous Coward (not verified) on 02 Dec 2009 #permalink

Aren't rainbows an optical effect of water droplets acting as a prism? Not the same thing.

If water droplets were retro-reflectors you would see an image of the sun, not a rainbow. Which, upon thinking about it, for the glass balls to work, the refractive index of the glass must bend the light 45 degrees, right?

Welcome to ScienceBlogs. Looking forward to following you here.

However, your pictures don't seem to be showing up. Don't know if it's my fault or not...

your image urls appear to be broken. or is it just me?

ah!, there they are, much better!

Welcome to Sb.

Years ago, I used to use a material made by 3M called Scotchlight in my visual effects work. Along with reflecting a light back to its source, it would do the same for projected images. This was the basis of an old, in-camera compositing scheme called front projection. The visual effects house I worked at had an optical system they called IntroVision that largely eliminated the normal visual artifacts of front projection techniques to create a very clean composited image shot in camera.

If you drive around at night and look at highway warning signs, you will see they also use a similar material. Same holds true for the "DOT Stripes" on the sides of semi truck trailers and the "safety stripes" on those safety vests often worn by crews and police working out on the roads.

Another neat thing is if you are on a freeway heading directly into the sun just before sunset, look into your rear view mirror, you will see those retroreflectors lighting up bright red.

Scotchlight - very cool stuff.

It is entertaining to shine a laser pointer at a stopsign three blocks away and watch it light up. And here's a pic of my bike's nighttime reflectorization using the camera flash on its lowest setting.

I've always wanted to take three hard drive platters and mount them as reflectors the way you suggest. Will have to try that now.


Great pic and good idea about the hard drive platters. I have been taking apart old 10 gb drives lately and those platters are great mirrors. Since they are round, probably be best to cut the corner out of some stiff cardboard box to mount them on.

I remember a physics post-doc talking about phase conjugate mirrors once. Is this essentially the same thing?

By mikey.duhhh (not verified) on 01 Dec 2009 #permalink

Great post about the hard drive platters! One of my Professor was discussing about this in one of his classes. I am a college sophomore with a dual major in Physics and Mathematics @ University of California, Santa Barbara. By the way, i came across these excellent physics flashcards. Its also a great initiative by the FunnelBrain team. Amazing!!!

"...on a pair of running shoes"

For a moment I thought that was a squid...

And there is no need to ruin your hard disk drives. Just switch to Linux and use your obsolete Windows CDs as mirrors. They are easier to cut to squares than glass platters.

By Lassi Hippeläinen (not verified) on 01 Dec 2009 #permalink

3-sided metal reflectors, known as 'corner reflectors' are used in radar and lidar, giving a superb return for so small a cross section.

rainbows are giant retroreflectors. you can only see them when the sun is low in the sky. no noontime rainbows!

The 3M Company invented a flat retroreflective material for use in the movie industry. It is now used in many industrial measurement applications.

By Jay Goldfarb (not verified) on 03 Dec 2009 #permalink

As it happens I have a good supply of platters from disassembling a bunch of old hard drives. Wonderful magnets, you see, from the voicecoil actuators. Usually I just give the platters to kids to play with.

The corner of a box sounds like a great way to align three platters. Will try that!