Reflecting in a Stream

Total internal reflection traps three lasers in a stream of falling water. When light tries to pass from the water – a material with a high refractive index – to the air – which has a lower index of fraction – it can only do so if its angle of incidence is smaller than the critical angle. Here, the light impacts the water-air boundary at a large angle and rather than refracting across the interface – like the distorted view of a straw in a glass of water – the laser light is completely reflected. Instead of escaping, the laser light is trapped, becoming a ribbon of light that swirls inside the water stream until the light is diffused. (Image credits: L. Yarnell et al.; F. Batrack et al.)

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