Light travels in straight lines, reflecting off obstacles. Fibre optic cables allow light to travel in any direction, bending around corners in a path set by the user. They are widely used in telecommunications and in medicine. Fibre optics rely upon the process of total internal reflection. In the Suitcase we use fibre optics to connect the optical apparatus for convenience, and also supply equipment to demonstrate this important process.
A fibre optic cable behaves similarly to a hosepipe, guiding the light down a specific path. Water can behave in a similar way to the cable, and is easily demonstrated. In this experiment a jet of water is lit up by a low-voltage LED placed behind the outlet hole. As the water flows out through the jet the light is constrained within the stream making it appear lit up. The light only escapes from the jet as the stream is broken, for example as it forms droplets or if a hand is placed in it.
Light guiding can also be demonstrated using a Perspex rod illuminated by an LED. The Perspex rod is flexible, and is a better representation for the behaviour of a fibre optic cable than the water, as the user has some control over the direction of the light. In this demonstration the end of the rod is seen to be brightly illuminated, but also flaws in the surface of the rod cause light to escape along its length. Light can also be seen to escape at points where the critical angle criterion is not met, for example if the rod is held tightly between the finger and thumb the higher refractive index of the skin allows light to escape.