Place the square block from the Black Board Optics set on a white piece of paper with the frosted side towards the audience.  Point a laser straight down into the block and turn it on.  The laser beam reflects off the paper and into the higher density medium of the block creating the cone of light you observe.  This cone describes the critical angle in exactly the same way as if you were at the bottom of a pool of water and looking up and out of the pool.  You would be able to see out of the water inside the cone, but would see nothing but a mirror effect if trying to look outside that critical angle.