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!
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!
