Water Light Guide

Procedure

A 2+ gallon tank or bucket with a hole (about .5-1.0 cm) near the bottom of one side is filled with water.  If the tank is opaque, then there should be a plate of glass or plexiglass installed on the side opposite the hole.  A container should be placed to catch the water that will stream from the hole.  A laser is aligned with the hole, illuminating the stream of water.  Be sure that the audience should be viewing the stream for the side, not head on.  As the water level in the tank decreases, the stream will bend at a sharper angle until the condition for internal reflection is violated, and bright spot will appear on the opposite wall. 

For the best effect, use an opaque tank, so that no light emanates from it.  Fluorescein or other fluorescent substance can be added to the water for increased visibility.  To convince the audience that the stream really is water, move your hand in front of it (beware of splashing!).

Explanation

Water has a refractive index of 1.33, so the critical angle for a water-air boundary is arcsin(1/1.33)=49 degrees.  As long as the water stream does not bend at an angle sharper than 49 degrees, the light will be totally reflected in it, illuminating the water stream.  When a part of the stream bends at an angle greater than 40 degrees, the light will "escape" the stream.

Suggested Audience

This demonstration is appropriate for low to mid-level audiences of all sizes.  For a larger audience, the lights in the room may be dimmed to make the light in the water stream more visible.

Shock Value: 3