Clamp the spring to the lecture table and then step back. When you hold the other end with some tension and shake the end with various frequencies, you can illustrate transverse waves traveling along the spring.
You can move your hand to generate a pulse or wave in the spring. Because of the clamp, the spring acts as a medium with one free end and one fixed end. By changing how far and how fast you move your hand, I can generate different amplitudes and frequencies. If you move my hand farther on each swing, you create a wave with a greater amplitude – the height of each peak is greater. If you move your hand up and down faster, you create a wave with a greater frequency – the number of peaks within a given length is greater.
With practice, you can also find the natural frequency of the spring and set up standing waves.
Engagement Suggestion
• Ask students: “Now that we’ve seen some features of transverse waves, let’s try an experiment. I’m going to send a single upright pulse down the spring. What will happen when it reaches the fixed end? Will it stop entirely, bounce back in the same shape, or bounce back upside-down?”
• “The pulse returns upside-down!”
Background
A transverse wave is one where the direction of oscillation is perpendicular to the direction of propagation. The up-and-down motion of the spring that forms each pulse is at a right angle to the forward movement of the wave. When a transverse pulse reflects off a fixed end, it returns inverted. If instead it had reflected off an open end, it would return upright. We can see this most easily with a single pulse, but this is true of a repeating waveform as well. We see mechanical transverse waves in springs, ropes, and other objects routinely. But another type of transverse wave surrounds us all the time – electromagnetic waves, like light, are transverse waves.