Description: This demonstration shows total internal reflection through a stream of water.
- Jack stand
- Laser waterfall container
- Powdered milk
- (Optional) Pyrex beaker
- (Optional) Red food coloring
- With the cork in, fill about 2/3 of the container up with water.
- Mix in a small amount of powdered milk, to make the light inside of the water visible.
- Place the container on an extended jack stand. Position it so it aims at the sink.
- Aim the laser straight through the container at the cork.
- (Optional) Fill a Pyrex beaker full of water, add a drop of green food coloring, and place it in the sink without students seeing.
- Turn on the laser. Double check to make sure it’s lined up on the cork.
- Lower the lights.
- Pull the cork plug out; the water should start streaming out of the container, and the light should be bending with the stream.
- Turn off the laser and replace the cork into the container when finished.
- (Optional) Pull up the beaker with green food coloring. You can say you trapped all the laser light, but you should probably clarify the gag afterwards.
- Only a small pinch of condensed milk is enough to show the laser through the stream. Adding too much to the water can prevent the laser from showing up in the stream.
- Corking the waterfall can be a messy affair; picking up the waterfall and corking it over the sink is recommended.
- The demonstration works with both red lasers and green lasers, but the green lasers tend to work better, and looks cooler, frankly.
Concept: This demonstrations shows the optical phenomenon of total internal refraction. When the laser strikes the surface of the water stream, the angle of incidence is greater than the critical angle, which causes the light to completely reflect inside of the stream. When the light reflects, it hits the other side of the stream, reflecting to the other side, and so on and so on through the stream, essentially trapping the laser inside of the stream.