Welcome back! This week, we’re taking a lot a three different demonstrations that are very valuable for introducing students to optical refraction. Refraction is the process by which light bends as it passes from one medium to another. Consider demonstration L4-03, with a metal rod sitting in a small tank of water; depending on the angle you view it from, the rod may seem to bend or break at the surface; there may even appear to be more than one of it, if you look in through the corner of the tank! And in demonstration L4-06, you can see a laser beam enter a similarly-sized tank of water, changing direction as it passes through the water’s surface.

a laser beam bends as it enters water, and a metal rod standing in water appears broken when viewed from an angle

The degree to which light bends depends on the difference in the index of refraction of each medium, which relates to the relative speeds of wave transmission in the medium. Prisms, lenses, and many other optical devices rely on carefully chosen angles and indices of refraction to create optical effects.

This public domain animation from Wikipedia does an excellent job of illustrating the effect. As the waves pass from one medium to another, the change in propagation speed induces a change in angle, as seen here.

Refraction animation by ulflund, wavefronts change direction at an interface

 You can see this happening on a larger scale in demonstration L4-01. Three rays of light are directed through a heavy slab of transparent plastic. The index of refraction of the plastic is much greater than that of air, so the light bends. In the photo below, you can see the beams change direction as they enter and exit the slab.

rays of light bend as they enter and exit a clear plastic slab


You can try it for yourself at home with this simulation in the PhET Interactive Simulations Collection: