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A laser beam in a stream of water. A laser beam is directed into a stream of water pouring from a hole in a plastic bottle. As the light tries to pass from the optically dense water to the less dense air, it is bent (refracted). At a certain point, called the critical angle, the light bends so much that it bounces off the internal boundary of the water stream. This phenomenon is called total internal reflection. In the narrow stream of water the light is continually reflected off the water-air boundaries. It is trapped in the curved stream, which appears to act as a light pipe.
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