Before starting the experiment, explain the different ways of categorizing waves and how light waves fit in these categories. First describe the difference between electromagnetic and mechanical waves. Explain that light, as an electromagnetic wave, can travel in a vacuum, while mechanical waves such as sound, require air, water or some other medium to transmit energy. Also describe the difference between transverse and longitudinal waves. Light, as a transverse wave, transfers energy in a direction perpendicular to the disturbance responsible for creating the wave, while longitudinal waves transfer energy in the same direction as that disturbance.
Use a Slinky to demonstrate how transverse waves, such as light, travel. If performing this demonstration in the classroom, push the desks aside to create a clear aisle from the front to the back of the room. Otherwise, move into the hallway. Tie a Slinky to a fixed point at floor level and stretch it back along the floor until it extends five to seven yards. Keep the Slinky flat along the floor. Hold the Slinky and snap your wrist to either the left or right, approximately one foot, before returning it to the original location. Note how the resulting wave travels down the length of the Slinky while producing hill like bumps perpendicular to the motion.
Instruct students to predict what will happen when two opposing light waves with pulses moving in the same direction meet. Write the different hypotheses on the board and take a class vote on which seems the most likely before testing these hypotheses using the Slinky. Stretch the Slinky across the floor until it extends seven yards in length. Have two students hold the Slinky, one at each end. Instruct both students to send a large transverse pulse through the Slinky, having both pulses travel along the Slinky in the same direction. Ask the class to describe the constructive interference that occurs.
Instruct students to predict what will happen when two opposing light waves with pulses moving in opposite directions meet along the same path. Gather the different hypotheses and take a class vote on which seems most likely. Have two students hold an outstretched Slinky, one at each end, and instruct both to send a transverse pulse through it. This time, however, have one student send a pulse moving in one direction while the other sends a pulse moving in the opposite direction. Afterward, instruct students to describe the resulting destructive interference and to analyze what would happen if the waves were transverse light waves.