A light source with a cylindrical lens is stuck to the blackboard by means of a suction cup. The source emits a beam of light that travels parallel to the blackboard. Similar suction cup holders hold a parellel sided piece of 3/4 inch thick clear plastic in the beam, showing that the bending of the beam toward the normal on entering and away from the normal on emerging is the same. A prism shaped piece of plastic may be inserted to show how the direction of a light beam may be changed.


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