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Total Internal Reflection in Water "Bucket of Light"

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 Physics   |   Science
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A beam of laser light can be trapped inside a stream of water by suffering total internal reflectionthe aquatic equivalent of a fiber optic cable. In our setup we have a 2-liter soda bottle with a hole cut into the side. The bottle is filled with water and a few drops of milk. A 5 mW HeNe laser is brought up to the side of the bottle, so that the beam exits the bottle via the hole. The laser light undergoes total internal reflection as the stream of water falls into the bucket in a cohesive stream.

Some of you may notice that the white bucket glows with the same color as the laser light, even after the water has stopped flowing into it! How can this be? Does the light keep reflecting around inside the bucket? Did we somehow find a way to violate conservation of energy? This part of the demo is a trick, of course: a network of red LEDs line the bottom of the bucket!

For more details on how we set up this demo, see
https://sciencedemonstrations.fas.harvard.edu/presentations/bucket-light

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