======Total Internal Reflection in a Water Stream======


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**Materials: **{{$demo.materials_description}}\\
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**Safety: **{{$demo.safety_description}}\\
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**Alternative titles:** Bucket of Light, Laser in Water Stream
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====Summary====                   
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====Procedure====  
  - 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.  
  - Place the bottle on a support so a container underneath can catch the water.  
  - Fill the bottle with water to above the hole, then unseal the hole so a smooth stream flows into the container.  
  - Aim a laser pointer horizontally through the bottle so the beam exits through the hole and into the water stream.  
  - Darken the room slightly and observe the beam “trapped” inside the stream, bending as the water falls.  
  - Move the laser slightly to optimize brightness and continuity of the guided beam.  




====Links==== 
Total Internal Reflection in Water "Bucket of Light" - Harvard Natural Science Lecture Demonstrations: 
{{youtube>XrWB0KLXpn8?}}\\

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



====Variations====
  * 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.



====Safety Precautions====
  * 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.




====Questions to Consider====
  * 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.)