What it shows:
A beam of laser light can be trapped inside a stream of water by suffering total internal reflection—the aquatic equivalent of a fiber optic cable.
How it works:
A stream of water flows from a hole in the side of a soda bottle (figure 1). The critical angle of 49° is such that total internal reflection will occur in the stream even when it is reduced to almost a trickle. Imperfections in the stream (and scattering agents added) allow some of the light to escape, and the effect is seen as a sparkling red waterfall.
The water reservoir is a 2L soda bottle, and the stream emanates from a 5mm hole just above the base section (about 8cm from the bottom). To ensure a smooth flow, a hole larger than 5mm is cut in the plastic, and a piece of clear tape stuck across it - the hole being made using a regular hole punch; this avoids unclean edges that occur when the plastic is cut and the thin wall produces a more laminar flow.
figure 1. total internal reflection within stream of water
A plastic bucket is used to catch the water, but it can serve another more covert purpose. A flashlight with a red filter can be hidden in the bucket (which should preferably be white plastic).1 As the pipe of red water arcs down, the lecturer secretly turns on the flashlight, and the whole bottom of the bucket begins to glow. Even when the water stops flowing, the bucket remains alight. You've captured all the laser light! Questions are invited from the audience...
Setting it up:
The laser, a 5mW HeNe, and bottle can both be mounted on the same lab stand attached to the edge of a bench (the bottle sits on a platform). For best results, the laser presses against the bottle opposite to the hole. The bottle is filled with water, with a small drop of milk to enhance the scattering, and a rubber bung used to cork it. The water will flow when the bung is removed. A dark backdrop is required (usually supplied by the blackboard). The catch bucket should begin with sufficient water in it to cover the head of the flashlight. Do a test "run" to ensure the bucket is in the right place!
The sneaky flashlight trick is easy to pull off because the hall lights are down, but you may want to keep the bucket behind the bench until the lecture starts. It fools 'em every time and always arouses a round of applause!
1Currently we use an array of red LEDs.