Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Fluid 'light-pipe'

  1. May 29, 2006 #1


    User Avatar
    Gold Member

    Hi, all. I need an optics specialist here. I seem to recall from many years ago that under the proper conditions, water can behave to some extent like a fibre-optic conduit. This might have been a misinterpretation on my part, but if it's true I have a use for it.
    Someone gave me a table-top fountain that is similar to a slab of sandstone with roses on the front in bas relief. There's a trough hollowed out on top, which the pump fills to overflowing. The water then runs down over the front. I mounted 3 high-intensity red LED's shining down, so the roses shimmer red, but it's kind of ugly since I had to shield them from direct view behind a sort of internally reflective hood. There are also a couple of green ones shining up from the reservoir to illuminate the stem and petal parts of the roses. Those are okay, because they're below line-of-sight.
    What I really would like to do, if it's possible, is to mount the red ones inside the top reservois and let the water itself channel the light down to the flowers. Does anyone know of a way that I can do that?
  2. jcsd
  3. May 29, 2006 #2

    Andrew Mason

    User Avatar
    Science Advisor
    Homework Helper

    What you do is take several lengths of clear optical fibre and let the fountain run over them so it looks like water and then shine light through the fibre.

  4. May 29, 2006 #3


    User Avatar
    Gold Member

    Thanks, Andrew. I thought of that, and basically like the idea. The only problems are that I'd need an awful lot of them (this thing is about the size of an encyclopaedia volume), and that I can't figure out how to make them invisible when the water isn't running. The closest that I've come up with is to maybe try hiding them under a thin layer of varnish. It might work, but then the material wouldn't look like natural stone any more. :frown:
    Might there be some fluid other than water, such as glycerin, that will conduct light and can still be pushed 15" vertically by an aquarium pump?
    Last edited: May 29, 2006
  5. May 29, 2006 #4

    Claude Bile

    User Avatar
    Science Advisor

    Water can indeed act like a fibre optic conduit, however you need to ensure a couple of things for it to work properly.

    - Ensure the water is under high enough pressure than it squirts outward a fair distance, this ensures that the curvature of your 'waveguide' is not too large.

    - Ensure there is no turbulence, as turbulence will greatly increase loss due to scatter.

    When I do this demonstration, I use a HeNe laser, so your light source may be one reason why it isn't working. Typically you need a lens for good coupling with a high-divergnece source such as an LED, though this is hardly a typical application, so I'm not 100% sure a lens is necessary. One possible alternative are those laser aligners tradesmen use, you can get from a hardware shop for about twenty bucks, they are big and ugly, but you may be able to extract the innards for your own use.

  6. May 29, 2006 #5


    User Avatar
    Gold Member

    Thanks, Claude. I don't think that I can eliminate turbulence, but that might be a good thing. I was rather hoping that the 'conduit' would break up upon splashing against the bas relief, and release the light at that point.
    One of the LED's just packed it up (:grumpy: ), so I'm not adamant about continuing their use ($12 for 4 of the stupid things; they're made for cars).
    Your mention of lasers definitely has my attention. My boss gave me a pen with a red laser as well as a blue LED for checking the authenticity of paper money. It was a spur-of-the-moment shining of the laser across the waterfall that gave me the idea in the first place. My boss paid about $25 for the pen, but W works in a "Dollar Store" where they sell them for $2.50. With her employee discount, that's $1.25 each. Basically, that means that I can get a couple of dozen if I give up beer for one night. :biggrin:
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?