Demos: 7A-25 Pouring Laser Light

This is a novelty demonstration but illustrates the concept of total internal reflection very dramatically. A cylinder of water with a small hole near the bottom is placed on a stand. The hole is plugged at first with a small cork and a red laser beam is directed from behind at the opening. When the cork is pulled out, a stream of water is ejected and the laser light is internally reflected in the stream. However, turbulence causes much of the light to exit along the way, producing a bright red stream of water. As the stream projects less and less the color becomes even brighter.

Directions: Dim the room lights. Place the cork in the hole and position the cylinder so that the stream can be passed into the sink. Fill the cylinder with water (best done earlier to reduce the air bubbles). Carefully aim the laser beam at the end of the cork. Pull the cork and let the stream play itself out.

Suggestions for Presentation: This demo is best done in conjunction with 7A - 24. Prepare the students with some banter about pouring laser light into the sink. Do the demo. It is very dramatic and will produce “oohs” and “ahs” from the class. Point out that the water appears red all the way down the stream because of the turbulence, i.e. if there were no scattering sites in the water, one would not be able to see the stream.

Humorous ending: Prior to performing the demo (preferably before class starts), place a beaker of water colored with red dye in the sink. Then talk about how the university budget has made us all cost conscious and that we can’t afford to be wasting the laser light. Tell them that you have recovered it to be used again. Then lift the beaker of colored water from the sink!

Applications:

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