Waveguide

How light stays on course, or total internal reflection

Press the button on the exhibit and watch the green laser beam inside the transparent rod. You may be surprised that light does not have to travel in a straight line!

Under certain conditions it can bounce off the walls of a material in such a way that it stays inside and travels on, as though trapped in a transparent tunnel.

This phenomenon is called total internal reflection. It occurs when light strikes the boundary between two media — here between acrylic glass and air — at a suitable angle. The beam is then not transmitted outwards but reflected back inside. By changing the angle of incidence you can watch its path change. Exceed the critical angle and the light escapes from the rod, and the reflections disappear.

This is exactly the principle behind optical fibres, which today carry vast amounts of information around the world. Light signals travel great distances inside them thanks to thousands of reflections off the fibre walls. Without this discovery there would be no fast internet, no modern telecommunications and none of a whole range of medical and industrial technologies.

Optical fibres carry more than data, though. At the Institute of Photonics and Electronics of the Czech Academy of Sciences we develop new types of optical fibre for use in lasers and sensors. We also use optical networks to transmit and distribute precise time. The national time scale UTC (Tempus Pragense) is produced by caesium clocks as a physical prediction of Coordinated Universal Time. The institute has contributed to it since 1970 and is thus among the facilities that help maintain the time standard used worldwide.

The clock near this panel is a reminder of how important precise time is for modern society. Without it, telecommunication networks, satellite navigation, power grids and many scientific measurements could not work reliably. The development of the national time scale UTC (TP) is associated above all with Vladimír Ptáček and Jiří Tolman, pioneers of the field at the Academy of Sciences.