Desktop Rainbow

Discussion in 'Physics' started by LuckyDog, Oct 9, 2011.

  1. LuckyDog

    Thread Starter New Member

    Apr 10, 2011
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    I have a educational background in quality control and business administration - both associates degrees. I am interested in electronics and have a General amateur radio operator's license.
    I've often wondered if a rainbow in a bottle could be constructed using a something like a LED projecting a light source in the 340nm range into a desktop container that is filled with atomized water or salt water (more dense cloud generated). I can only speak in generalities, not knowing the physics of light that may be in play here. I have worked around xenon arc accelerated weathering equipment, and know a little about the light wavelengths used to provide the most intense part of the light spectrum exhibited by the sun.
    Has anyone ever experimented with anything similar in your studies or experience??
    thanks.
     
  2. someonesdad

    Senior Member

    Jul 7, 2009
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    Welcome to the forum, LuckyDog!

    It's an interesting idea, something I had never considered before. But there will probably be some challenges. First, LEDs aren't really usable, as they are narrowband devices (typical FWHM of 10-20 nm). You probably want a broadband light source such as a fluorescent tube or incandescent lamp.

    Second, I suspect a mist wouldn't be quite as good as a collection of raindrops. I'd imagine that the diameter of typical raindrops would be in the 0.1 mm to 5 mm range. It would probably be difficult to get 1 mm or larger drops in a bottle. I don't know anything about the required meteorological conditions, but I'm guessing you want to use the larger drops. Certainly it's something that could be determined by some experimentation and you'd probably have fun doing it.
     
  3. steveb

    Senior Member

    Jul 3, 2008
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    If you can make a rainbow with a garden hose and a nozzle in the sunlight, it seems to me you can make one on a table top. Personally, I wouldn't try to do it in a bottle, at least not at first. One complication is condensation on the inside the bottle.

    I would go for an open tray design myself. A shallow tray can hold the water and a hidden pump can pump the water up to a spray nozzle above the tray.A highly directional white light source (full spectrum LED or incandescent would work) would need to be aimed down making a 40-53 deg angle to the viewer. Use an atomizing sprayer to make a wide and flat plane of water droplets aimed down into a tray (almost like a waterfall). You can probably see a double rainbow with this approach. The primary rainbow is an an angle of 40-42 degrees and the secondary rainbow is at a 50-53 degree angle.

    It wasn't clear why you suggested 340 nm wavelength. Are you trying to excite a medium with ultraviolet light, and then generate colors through fluorescence?
     
  4. LuckyDog

    Thread Starter New Member

    Apr 10, 2011
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    I calibrate instruments used in the coatings industry, and have access to some very speicalized items. The 340nm comes from the insturment that delivers proably the most destructive portions of the sun's spectrum to advance the test exposure time to the minimum. I thought it might be the brightest also, maybe an advantage to a making a scale model of the sun.

    thanks for the warm welcom........
     
  5. LuckyDog

    Thread Starter New Member

    Apr 10, 2011
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    I may have gotten my replys mixed, but thanks for your help. I may have a solution to the droplet size. In grinding pigments for coatings some of the mills use glass media...small bb-size clear, glass balls. I'm wondering if a small wall of these would provide a waterfall type of a prism only dry, that a light beam could be projected through. Thanks for the info on droplet size and light angles.
     
  6. steveb

    Senior Member

    Jul 3, 2008
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    It seems that this would just be dangerous to use the UV directly and it wouldn't give you the part of the sun's spectrum you need. You need a continuous spectrum in the visible band to make a rainbow in the traditional way, and broad spectrum white light sources are a simple and effective way to get that.

    I expect that the UV light could be used to generate a spectrum of colors by some method, and that would certainly be interesting physics and engineering at work. (For example, broadband LED sources us a phosphor to generate a wide and balanced spectrum from blue light.) But, if you just want to make a nice conversation piece, then simple available light sources are the way to go.

    Keep in mind that fiber optics could be used to deliver a light beam in an elegant and unobtrusive way. Or even a solid glass rod can act as an effective waveguide.

    That's an interesting idea to use glass bb's instead of water drops. Keep in mind that the proper refraction requires a good air/glass or air/water interface with a spherical shape. So the bb's need would to be suspended somehow without interfering with the light refraction.

    If you succeed, please post some pictures here someday !
     
    Last edited: Oct 10, 2011
  7. someonesdad

    Senior Member

    Jul 7, 2009
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    Note that raindrops are not spherical -- and they certainly don't look like the stylized teardrop drawings you see everywhere. They look more like hamburger buns. One of these days I'm going to set up a strobe and actually get some pictures of some to satisfy my own curiosity (and measure some typical velocities).
     
  8. steveb

    Senior Member

    Jul 3, 2008
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    OK, that's a good point. Smaller raindrops approximate spheres reasonably well, but the approximation breaks down when they larger than a certain size. It seems they become hamburger buns at midsize, and then, when they get even larger, they start to look like parachutes.

    Also note that the derivation of rainbow angles is based on the spherical assumption usually, and the approximation seems to give good correspondence with real rainbows. I don't know if that means rainbows are always made of small raindrops, or if hamburger buns and parachutes also happen to give similar angles.
     
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