This week’s Demonstration of the Week is L5-11: The Laser Waterfall. Check it out in this video below, presented by physics student Ela Rockafellow, a leader of our Society of Physics Students.



The laser here is illustrating internal reflection: depending on the index of refraction of the water and the angle the light hits it at, more light can be reflected back and forth within the stream of water than passes through it. At a certain point, it exhibits total internal reflection, where essentially all of the light is traveling along the stream rather than heading straight out the side.

You can experiment with this phenomenon in this TIR simulation from Boston University. You can change the index of refraction of the outside medium (usually about 1 for air) and the interior (about 1.333 for water, around 1.5 for most glass, for example. Try different combinations and see whether the light stays in the stream or passes outside!

This is the same physics that lets fibreoptic cables carry signals over long distances – the light stays within the cable, traveling with very little loss, and allowing us to operate much of modern communications technology.