Total Internal Reflection in a Water Stream

A laser beam is sent through a small hole in a water-filled bottle so it enters the flowing stream. The light undergoes total internal reflection and follows the curve of the stream, just like light in an optical fiber.

1. Make a small hole in a plastic bottle about 5 cm above the bottom; keep the hole sealed (e.g., with tape or a finger) until ready.
2. Place the bottle on a support so a container underneath can catch the water.
3. Fill the bottle with water to above the hole, then unseal the hole so a smooth stream flows into the container.
4. Aim a laser pointer horizontally through the bottle so the beam exits through the hole and into the water stream.
5. Darken the room slightly and observe the beam “trapped” inside the stream, bending as the water falls.
6. Move the laser slightly to optimize brightness and continuity of the guided beam.

Total Internal Reflection in Water “Bucket of Light” - Harvard Natural Science Lecture Demonstrations:

📄🎞️ Description or Title - Collection of Physics Experiments: https://physicsexperiments.eu/1765/total-internal-reflection-in-a-stream-of-water

• Add a few drops of milk to the water to scatter light and make the beam path easier to see.
• Try different hole sizes (slightly larger than the beam) to compare how cleanly the beam couples into the stream.
• Compare with a straight acrylic rod or clear tubing to connect the demo to commercial optical fibers.
• Test different laser colors (red vs. green) and note visibility differences.

• Never look directly into the laser or point it at people; avoid reflective surfaces at eye level.
• Secure the bottle and keep all equipment over a catch basin to prevent slips from spills.
• Use a plastic bottle (not glass) to avoid breakage; check for sharp edges around the hole.
• Keep electronics and power strips away from water; dry any spills immediately.

• Why does the beam follow the stream instead of escaping into the air? (Angles at the water–air boundary exceed the critical angle, so total internal reflection occurs.)
• How does Snell’s law predict the critical angle for a water–air interface? (Set the refraction angle to 90° and solve for the incidence angle.)
• Why does adding milk make the beam path more visible? (Small particles scatter light, illuminating the beam inside the water.)
• What properties of optical fibers mimic this demonstration? (High-index core, low-index cladding, and light entering at angles that meet total internal reflection conditions.)
• What happens if the stream breaks into droplets? (Reflection path is lost between droplets, so the beam leaks out and visibility decreases.)