bicycles, lunar lasers and cube corner retroreflectors

I have a laser. A pretty powerful one so much so that I think it really would cause damage if it was aimed at my retina for a few seconds. It’s a green laser that I have used to aim at stars. You can see the beam at night time which makes it easy to follow compared to a finger or the cheap powerpoint lasers that people use in meetings.

I’ve aimed it at street signs too and what a powerful reflection I got. Almost blinding light. And probably not a good idea to do that…much. So would an ordinary red pointer laser do that? Probably. Maybe you wouldn’t get quite as bright a reflection but I’m sure it would be pretty bright. The laser that’s used at the Apache Point Observatory APOLLO project beams a laser to the moon….off a street sign there. No, it’s not a street sign. But dang, it works like a street sign. And it works like a bicycle reflector too.

All this reflective stuff comes from a neat little design called cube corner reflection. They can be made out of cheap plastic or prisms in glass or plastic and obviously can be in red color like the bicycle reflectors. The kind on street signs and on the very reflective tape have tiny beads that do this retroreflection. Some other designs can have coatings on the back to keep light from leaking out.

Here’s the simple explanation of why light is reflected. It’s not a flat or curved mirror because that would only work if you aimed it straight at the mirror to get the reflection. These little cube corners will take a light from a wide range of angles, bounce it off the inside corner of the cube back out at the exact same angle/direction that the beam came in. Meaning, you don’t have to be directly in front of the reflector for this to work. The angles inside the corner will take care of sending it back out to you.

So what if I aimed my green laser at the moon. Could I get a reflection off one of those retroreflectors the astronauts put there? No. The laser at the Apache Point Observatory is quite a bit more powerful, like gigawatts. My laser is 50 milliwatts I think. And the detector back at the observatory might only get one photon back. Our eyes wouldn’t even know one photon from another from looking at the moon.

If you bounce a small rubber ball into a corner, you should get the ball back at basically the same position that you threw it. Of course, how hard and what kind of ball and the fact it’s not a light beam won’t be exactly the same thing.

So for those bicycle riders that are now disappearing in the dark because the sun’s going down early, check those reflectors. You could also put some extra reflector tape on you and your bike. Here’s a website where you can get some…

Info about the APOLLO project:

APOLLO laser

APOLLO laser

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