Water-Following Laser Beam

Whenever light passes from one material to another, it changes direction. This is referred to as refraction and leads to many surprising and beautiful effects.

One of these is total internal reflection: when light tries to pass from a material where the speed of light is slower (such as glass or water) to where the speed of light is faster (such as air), then as long as the light strikes the boundary at a shallow enough angle, all of the light is reflected (and none refracts through). This is one reason why most good binoculars redirect the light using glass prisms (surrounded by air) rather than using mirrors: even with the best reflective coatings, at least some light is lost.

So, a beam of light in a stream of water can stay trapped in that stream, following wherever it flows (as long as the angle between the light and the surface stays shallow enough). This has been used as a cool science demonstration for over a century and was first demonstrated in 1841. This effect makes fiber optics possible – in fact, if you make this demonstration, you’re using water as a fiber optic cable.

Most of these demonstrations are tricky or annoying for two reasons:

1. It's tricky to aim the laser beam exactly down the water stream.
2. The water stream tends to have rough edges, and break into smaller droplets, after flowing a relatively short distance (which allows light to escape earlier, making for a less dramatic effect).

Problem 2 can be mitigated in one of two ways:

1. Have the water pass through a very thin, perfectly round, sharp-edged hole.
2. Let the water flow along a tube (with a smooth, straight exit hole), such as a straw.

In Steps 1-3, I show an example using a thin round hole. This is faster and easier to make, and produces a very nice effect, so I encourage you to try it for your first attempt. It’s also described nicely by Harvard University’s Natural Sciences Lecture Demonstrations.

Steps 4 through 8 shows an alternative method where I cast an acrylic block to make it easier to align the laser, and attach a straw. This can make setup slightly easier; but since it's more work to build, only make it if you plan to use this demonstration frequently.

For the thin hole method, I used:

• A 2-liter bottle
• Packing tape
• A hole punch
• A laser pointer

Instead of a 2-liter bottle, you could use any clear container in which you can make a hole. (It helps if it holds a significant volume, and has an airtight cap.) And instead of packing tape, you could use thin transparency film, or other thin plastic. (It should be thinner than the walls of the 2-liter bottle; but not floppy like plastic wrap. And doesn't have to be clear, just waterproof - but having it clear is more attractive.) If you use a hole punch, it should be as sharp as possible to produce a clean, round cut.

For the acrylic block method, I used:

• A drinking straw
• Clear casting expoxy resin and a roughly 2-inch (~5 cm) cubical silicone mold
• A 2-liter bottle and lid
• A small plastic container
• A laser pointer

For the straw, I used one with a roughly 1/4-inch diameter. This is large enough compared with the laser beam that the laser alignment is at least somewhat forgiving. You could make a more attractive demonstration using clear acrylic tubing instead.

For the water container, I used a 2-Liter bottle: it’s easily available, inexpensive, watertight, and lightweight. But you could use any container, or a hose with a valve - anything that will allow you to connect to a supply of water (even a faucet).

First, I doubled the packing tape over on itself, to give something I could punch cleanly. Then I used the hole punch to create a circular hole. Under a magnifying glass, I could see that the punch wasn’t perfectly clean, so I used a sharp knife to make the edges as smooth as possible.

Then, I used that knife to cut a hole in the side of the 2-liter bottle (relatively close to the bottom, high enough that the walls were smooth and vertical), slightly larger than the hole from the hole punch. 

Finally, I cut a hole in a larger piece of tape, with the hole slightly larger than the clean round hole. I taped that larger piece to the 2-liter bottle, with the smooth round hole centered on my rough-cut hole in the bottle.

Fill the bottle with water, keeping your finger over the hole to keep water from flowing out. If you want to be able to see the beam while it’s in the water, shake in a few tiny flakes of powdered coffee creamer. When the bottle is full of water, screw on its cap to create an airtight seal.

When you take your finger away from the hole, a few drops of water will flow out, until the air pressure below the cap is low enough that no more water will leave the hole. (This is another interesting physics demonstration that you could spend time discussing.)

Using a binder clip, keep the laser’s button pressed in. Then, rest the laser on something (or a group of things) so that its beam is at the same height as the hole. Shine the laser through the bottle so that the beam leaves the bottle at the hole. 

When you unscrew the cap, water will flow out through the hole, and light will follow the stream. If you use the coffee creamer, you'll be able to see that the light follows straight-line paths between each reflection from the walls of the water stream.

At first, all of the light will stay in the water stream; it can be fun to put your hand in the water stream to see the light hitting your hand.

As the water level drops, the exiting water stream will bend sharply enough that some light will escape through the sides. If you do this into a white sink or bowl, you may see beautiful, intricate patterns, as slight variations in the angle of the water internally reflects some light, while letting other light leave the stream and shine into the bowl.

The remaining steps describe a different method that takes more time to build, but is slightly easier to use once built.

Rather than try to drill and tap something that could mount well onto a 2-Liter bottle, I took the lid from the bottle, and drilled a hole in it. Then (using hot glue), I glued it onto a piece of clear plastic.

As noted above, you could instead use a connector to any sort of hose, to connect to a faucet or any other water source.

I first poured around a half inch of acrylic casting resin into a cubical silicone form, and allowed it to mostly cure (while still being somewhat soft). Then, I pressed the adapter into that layer, and filled the rest of the form with resin, up to the level of the (inverted) 2-liter bottle lid.

I marked the block at the position where I wanted to place the hole, then drilled a small pilot hole using a smaller drill bit. Then, I took a ¼” diameter drill bit, and drilled the whole way through the acrylic block. (If your straw is a different diameter, you may need a different size bit.)

(While it would be possible to do this with a hand drill, a drill press is very much easier and gives a hole with cleaner edges.)

The straw will only go from the outside of the block to the inner edge of the adapter (less than halfway through). So why drill the whole way through? Alignment: the hole drilled for the straw serves as a pilot hole for the laser pointer, so the center of the laser is guaranteed to be in line with the center of the straw.

After drilling the hole for the straw, I took a ½” diameter drill bit, and drilled to a depth of about ½”. The laser pointers I used made a snug (but still slightly adjustable) fit with this hole

When water flows from the container, you want it all to flow out through the straw, with none flowing back to the laser. I took a piece of clear flat scrap plastic, used scissors to cut it into a ½” circle, and glued it (using silicone sealant) to the block, and allowed it to cure.

Insert the straw into the acrylic block, and press the laser into its hole on the other side. The beam should follow through the straw. But some laser pointers (especially red ones, in my experience) don’t have a beam that stays perfectly parallel to the shaft of the laser. There should be a little “play” in the fit that can allow adjustments.

With my block, the fit between the straw and acrylic block was tight enough that I didn’t need to use any sealant. If you want or need a better seal, then if you want the straw to remain permanently, you can use an adhesive (e.g. silicone sealant, rubber cement, etc.). If you want to be able to remove it later, you can use a gel (e.g. Vaseline).

If you’ve gotten it all to work properly, you should barely be able to see the laser beam in the water; but if you place your hand in the water stream (or allow it to break up into droplets), you’ll see a bright light. Or, you can use a tiny amount of powdered coffee creamer into the water, and mix thoroughly. If you’ve used the right amount, you won’t be able to notice any cloudiness in the water, but you’ll still be able to see the beam very clearly.

Enjoy!