green

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…http://www.brightthread.com/

Info about the APOLLO project: http://physics.ucsd.edu/~tmurphy/apollo/apollo.html

http://en.wikipedia.org/wiki/Retroreflective

APOLLO laser

APOLLO laser

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Fluorescein in your eye

Fluorescein in your eye

If you’re an eye doctor you know about this stuff. If you’re not, it’s ok to get fluorescein “die” in your eye from the eye doctor. I’m referring to the topically applied orange colored little strips of paper that kinda looks like those pH test strips. But both of you hang on because I have a few interesting things to say about this commonly used diagnostic chemical. It sounds horrible…chemical….but it’s not at all horrible otherwise we wouldn’t use it.

What’s interesting about these strips with fluorescein is that they fluoresce. And pray tell what does that mean? This is where I get to talk about quantum mechanics again! If you didn’t read the last post, it all started there. This little piece of paper, the fluorescein strip when touched to your eye, will turn a different color when blue light is shined on it.

So here’s what I’m saying. The color of the fluorescein is orange on the strip. It goes in your eye orange. Blue light is turned on. The fluorescein in your eye no longer looks orange. It’s green! Turn the blue light off and the fluorescein in your eye is orange again.

Here’s where quantum mechanics comes in. Easy lesson here. The blue light has a certain wavelength, it’s shorter, and thus has more energy. It hits the orange fluorescein in your eye. The little electrons in the fluorescein absorb that energy but a cool property of something that fluoresces is those electrons return some of the energy. And the energy we see is less than the blue wavelength. The color looks green! Green has a longer wavelength than blue. And this is the case with most things that fluoresce. The higher energy of light causes the fluorescing material to “reflect” a lower energy wavelength. You might wonder what happened to the rest of the energy. It is also released by those electrons that sent out the green wavelengths. The remaining energy is heat. A pretty limited amount so there’s no heat you can feel. There’s a measurement of the ability of a material to fluoresce which is called quantum yield and fluorescein has a high yield which kinda means we see the green reflection very easily.

Let’s just take a made up example of fluorescence. Say you shine green light at a material that fluoresces. What are the likely wavelengths it would “reflect?” Right, maybe yellow or orange or red. Those have less energy than green.

But not all things fluoresce enough for us to see, obviously. If they did the world would be very psychedelic. But there are a lot of substances that when a certain wavelength of light hits them, they send back a different wavelength that we can see. Black lights (aka, ultraviolet lights) are very cool because there are a lot of things that fluoresce from that wavelength of energy. There are all kinds of things that fluoresce from UV light like some rocks. I read on a website that an apartment property manager was going to take an ultraviolet light to the carpet of a renter to see if there’s any urine from pets there. Yep, urine fluoresces under UV light. Better not try to hide your pets. Property managers have their “secret” ways to find out!

fluorescent eye

green (yellowish green) is where the fluorescein is

fluorescein strip

fluorescein strip