Thursday, December 08, 2011

Thurs-Demo: The One with Frickin' Laser Beams

I've got a few demos cooking (or boiling up), but here's a quickie with the index of refraction. I used a ring stand and a few clams to hold some laser pointers above an aquarium.

Using Photoshop to measure the incident and refracted angles. Based on a reference I found online, the measured index of refraction compared to the expected values were off by less than 1%. Not bad for a quick and dirty setup. Check out how close I managed to get the lasers aligned, too.

On a side note several materials fluoresce in the blue laser light. So I put a few drops of fluorescent dye into the tank and waited for something interesting to happen.

Oh, and by the way - some of this refraction stuff is why you can get a "green flash" at sunset. You can find some more on the hyperphysics website at Georgia State and a detailed review of the phenomenon by Andrew Young.

When my wife and I were in Hawaii, I managed to see the green flash on three different evenings over the 10 days we were there - not a bad average. (darn, pics are on a different drive, so that will have to wait - sorry).

And now the flash:




  1. "..a few clams to hold some laser pointers.." That's funny.

  2. If you promise them a reward, the clams can be very cooperative. But if you renege on that promise, look out: they are the ones holding the laser beams!!

  3. Damn, that's a great typo. I'm going to leave it in there. :)

  4. clams with frickin' laser beams!! are they ill tempered?

  5. Wonderful demo! Just the thing I need for my lecture on Snell's law tomorrow, and I can explain dispersion too thanks to you!

    There is no apparent reflection of the blue laser. First of all, it's hard to
    get a blue laser (red and green are easier). Second of all ... no reflection,
    and also no scattering in the water (looks much cleaner). Did you photoshop that one in? I forgive you if so but wanted to know.

  6. Did not mean to doubt you, but could not understand the blue laser not reflecting. Then you said you used the blue laser to make things fluoresce. I need to think about this. Did you perhaps get lucky (or be intentional) and hit the critical polarization angle so that you got minimal reflection... or is it just that somehow you haven't got rough glass or steam or whatever it is the other beams are scattering off of over on that side of the tank.

    Anyway ... Nice!

  7. Hi pcardout,

    Photoshop was not used to create the blue. I think the blue laser was reflecting - it just wasn't bright enough for the camera to record what little light was scattered off the "fog" used to get the laser to be visible in air. if I increase the blue color saturation in photoshop, a faint line appears. It's just not reflecting off the surface as visibly (it's a weaker laser, so it's not as bright to begin with). But, the lasers were all in the same shot (I did turn the red and green lasers off halfway through the exposure because they were so bright).

    And the blue laser definitely causes fluorescence in some materials. I think I put a blog post up on it a little after this one. Maybe not. I have some pictures, so perhaps I could add some info.