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.

Here's a video of the Bucket of Light in action (https://youtu.be/XrWB0KLXpn8) See also: Light and Optics

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 is across it—the hole in the tape being made using a regular hole punch to avoid the unclean edges that occur when the plastic is cut, and the sharp, thin wall of the tape produces a more laminar flow than the plastic would.

Figure 1. Schematic diagram of the laser and soda bottle setup to show total internal reflection within stream of water.

A plastic bucket is used to catch the water, but it can serve a more covert purpose: A flashlight with a red filter[1] can be hidden in the bucket, which should preferably be white plastic to make the interior illumination effect more apparent. 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...

Photos of the Bucket of Light demo setup: The top two show the 5mW HeNe laser aligned with the hole in the 2L soda bottle; at bottom-left is a top view of the bucket with the LEDs illuminated; at bottom-right is a photo of the demo in action, with the internally reflected light scattering out to the side of the stream as it flows down into the bucket.

Setting it Up

The 5mW HeNe laser and bottle can both be mounted on the same lab post attached to the edge of a bench (the bottle sits on a flat Masonite platform). For best results, the laser should be about 1cm from the bottle, directly opposite from the hole. The bottle is filled with water, with a couple drops 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!

Comments

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!

[1] Currently we use an array of red LEDs that are automatically switched on whenever tap water is added to the bucket.