bicycles, lunar lasers and cube corner retroreflectors

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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Blue is hot and Red is cooler

Blue is hot and Red is cooler

What if I told you that blue means hot and red means cool? I know that’s not the way it looks on my sink handles. There’s a reason why I ask the question because most of us generally assume that blue means cold and red means hot. This is almost like a previous post of mine when I was talking about why the word right can mean so many different things.

This blue and being hotter thing has to do with stars as well as earthbound examples of blue being hotter than red. Stars are extremely hot, duh! Apparently a teaspoon full of sun will cause some devastating effects over hundreds of kilometers if you dropped that on our earth. But our sun is considered a cool (red) sun. If you ever look up at the sky, and I hope you do, the stars aren’t all white. Let’s throw out the twinkling ones because our atmosphere is causing that problem. I’ll take one star as an example….Betelgeuse (pronounced beetlejuice by most people). It’s a very bright star we can easily see. It’s red! It’s not as hot a star compared to Sirius, a bluish star. By the way, these stars are massive, way bigger than our sun. Put Betelgeuse in place of our sun and it will smother us and maybe extend out to Jupiter. Stars have different colors during their life spans. The really hot blue ones don’t live as long either.

Here’s an example of earthbound blue being hotter than red. A flame from a propane or butane torch is hottest at the tip when it’s blue and the lower part of the flame is red. There’s a bit more to burning (combustion) and colors but let’s just leave it at that (high frequency=high energy=high temperature=blue flame). So red is “cooler” than blue. I’ll include a couple of links of some more scientific explanation at the bottom if you’d like to understand combustion, temperatures and flame color. I sure do like wikipedia and you can see that from all the links below.

So how did blue become known as cool on our water faucets and red become hot? I’m just guessing at the reasoning here. Maybe it’s because a fire is generally reddish looking and the stove also turns red when it’s hot. And blue looks like the color in a swimming pool or the ocean? You got me. But are you down with that?:)


Hertzsprung-Russell diagram