Neon Waterfall

We include this segment to illustrate the internal reflection of light in a column of water that is “guided” downward by the force of gravity. The rays of light, traveling in a column of water in a darkened lab make for a stunning, “illuminating” example of the behavior of light.


We include this segment to illustrate the internal reflection of light in a column of water that is “guided” downward by the force of gravity. The rays of light, traveling in a column of water in a darkened lab make for a stunning, “illuminating” example of the behavior of light.

The laser in our segment is an inexpensive one that is designed for use as a level and sold in hardware stores and online. We use this particular unit in several experiments and models because it is mounted on a tripod. Further, the laser/level and tripod are attached with a threaded bolt that allows the unit to disassemble and store easily in a standard toolbox or shallow drawer.

In running this model, ask the observer to imagine the surface of a swimming pool from beneath it. Many will recall that it resembles a liquid mirror. Careful observers may point out the same optical effect in the tank if they are able to see the bottom surface of the water level (you may want to position the tank on a higher lab bench to facilitate this). A similar, yet cylindrical surface is formed as the water passes through the opening in the side of the tank, and that is the surface that reflects the light internally in the column of water. If possible, ensure that observers “follow” the path of light to the bottom of the sink or catch basin. The light remains in position on the bottom of the sink as the water splashes away, as though a flashlight’s beam is focused there.

What’s the matter here?

We add particles of powdered milk simply to give the rays of light some opaque matter to interact with. When light strikes the surface of matter, it can do one of three things (pass through, be absorbed or reflect). Milk, a combination of nutrients (sugars, fats, protein) in its powdered form, mixes in our tank and begins the process of forming a colloid. As some of the larger milk particles drift into the path of the laser beam in the tank, the amplified light striking them produces the “sparkling” effect that is filmed in our segment. Observers near the tank may comment on the dramatic effect of the light interacting with the milk solids in the tank. The milk sugars, of greater solubility, slow the laser down and allow us to better observe it as it travels with the bending column of fluid that is “released” as we remove the cork in the side of the tank. Other iterations of this model feature a “valve” constructed of masking or packing tape, yet we prefer the cork because it is easier to replicate the experiment several times in a row (the tape loses its adhesive properties as it becomes wet during the experiment). Simply stated, we are adding material to alter the composition of the medium (the matter through which our laser is passing through) to better observe the behavior of the light rays in our experiment. You may pose a challenge to your students; can they propose a better solute than the powdered milk? Test several suggestions.

Safety first

As the laser units in our segment are meant for home use, we are able to use them quite safely by following the safety precautions of the manufacturer (essentially to avoid aiming the laser directly into the eye). We position the tank and laser above or below eye level on the lab bench to keep the beam from reaching the eyes of the observer. A further precaution you may consider may include a set of low cost polycarbonate protective eyewear with a thin film coating to filter out harmful wavelengths of light energy, yet these are typically engineered for more powerful lasers than those sold for home and office use. Still, it’s a very good idea to have the eyewear on hand to teach the basic principles of lab safety when handling lasers, for those who choose to work with more powerful units at higher levels of study in physics and optics.


Draw an “X” on the inside of the cork to aid in focusing the laser on it. If you are unable to find a cork small enough to fit the hole you have made in the tank, you may whittle down a larger one or substitute a pencil’s eraser.

Project Time!

To build a bigger and better version of the tank, substitute a polycarbonate “Critter Cage” as shown in the beginning of our segment. In drilling the openings, use a tungsten drill bit and rotate it at a high speed (“high RPM setting”). Also, warm the polycarbonate tank with a hairdryer or in a warm water bath to soften it and to aid in drilling through it without risk of cracking the side of the tank (test your drill on a small piece of polycarbonate. I use the clear “trap door” on the top of the Critter Cage). I also recommend placing a large piece of clear packing tape on the side of the tank before drilling, to stabilize the material during the drilling and also to trap “burrs” of plastic that are produced in the drilling (wear goggles, as always, when working with a drill). You may omit the side lens and simply fire the laser through the polycarbonate itself, or you may use a simple microscope slide for the side lens and attach it with clear silicone sealant (inexpensively procured in the bath tube and tile section of your hardware store or the windshield repair section of an auto supply store). The silicone sealant may smell like vinegar as it dries because acetic acid is used as a curative in its formulation). The strong odor dissipates after the sealant has dried. It may seem obvious, but rest the tank on its side while the lens is being applied, to allow gravity to stabilize it until the silicone sealant dries. If you run a science or engineering club as an elective, ask your team to build a class set of the testing tanks, or perhaps one for each science teacher in your school who may wish to run the experiment. Building the tank itself makes for a fine fabrication lab for enrichment, and kids really enjoy the engineering of it.