Demonstration of total internal reflection. This is the effect responsible for light guiding in an optical fiber. In this photograph the beam from a diode laser (635 nm wavelength) enters a transparent acrylic block through its left side face. The beam is then totally internally reflected several times at the top and the bottom faces of the block, and then exits through the right side face. The acrylic block is specially formulated to scatter some light and make the beam path visible from the side.