Activity 1
Using Waves to Communicate

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Background Information

“The Wave” is a model of wave motion. It shows how the medium moves up and down as a pulse passes by. It is possible to imagine a more detailed model by picturing springs attaching one student to the next. Then as the pulse approached a student from the left, the student would feel pulled up by the stretched spring on the left. The reaction force would pull downward on the student on the left. Once displaced, the student would exert a force on the next student to the right. This more detailed model incorporates the restoring forces that bring the medium back to its equilibrium position. These forces are essential for wave motion through a medium.

The motion of a pulse can be the basis of a code. At a certain time, the reception of a pulse could code for binary one; the absence of a pulse, for zero. Alternately, a train of pulses could represent the dot and dash of Morse code. If the transmitter and receiver set up a convention that there will be one signal every ten seconds, and it will be either a single pulse or no pulse, then information can be sent over the spring. For more information about spring waves, see the Background Information section for Communication, Chapter 4, Activity 1.

In a similar way, information can be sent over a stretched string with the cup phones. In this case, of course, voice can be transmitted, but the quality of the transmission is low, so some sort of code would improve the accuracy of the communication, although at the expense of speed.

The ripples the student make in Step 1 are like a two-dimensional version of the Slinky waves. By symmetry, the ripple spreads out in the same way in all directions along the surface of the water. The water right in the center moves up and down, and this disturbance creates circular waves. As each wave passes the piece of paper, the piece of paper moves up and down but remains the same distance from the center of the pattern, just as the Slinky coils move sideways as a pulse passes but remain at the same place along the spring. Ripples could send a code, just like Slinky waves. In fact, a tsunami creates a small number of ripples, less than a meter high, which spread out like the ripples made in Step 1. Earth scientists read the “code” of these ripples to find out how the tsunami was formed. Nearly all waves require a medium. When the medium is disturbed from its equilibrium, restoring forces oppose the disturbance and bring the medium back to equilibrium. Only light waves travel without medium.