Wave Demonstrator

Introduction

Use this coiled spring to demonstrate wave types and wave properties.

Concepts

• Transverse waves
• Amplitude
• Frequency
• Standing waves

Materials

Safety Precautions

The wave demonstrator spring is safe if used in a normal manner for wave demonstrations. Only misuse of the spring is likely to result in potential injury. Protective eyewear should be worn when using the spring.

Procedure

1. Transverse Waves: Fasten one end of the wave demonstrator spring to an object (e.g., wall, table leg) with string or have a partner hold one end of the spring firmly. Shake the free end of the spring up and down once. Observe the pulse move down the spring (see Figure 1). Notice that the amplitude of the wave decreases as it moves the length of the spring. Now generate a train of transverse waves by shaking the spring up and down, respectively, while keeping the amplitude the same. Transverse waves displace particles perpendicular to the direction of wave propagation (see Figure 2). Next, increase the frequency by shaking the spring more rapidly while keeping the amplitude the same. Shake the spring at various amplitudes keeping the frequency the same in each case. Repeat trials until all the desired principles have been observed.

2. Standing Waves: You may also use the Transverse Wave Procedure to create a standing wave. A standing wave is a wave pattern that results when two waves of the same frequency, wavelength, and amplitude travel in opposite directions and interfere with each other. When a standing wave is created, you will see nodes and antinodes. A node is a point in a standing wave that appears to be stationary. This is due to complete destructive interference. An antinode is a point in a standing wave, halfway between two nodes, at which the largest amplitude occurs (see Figure 3).

Teacher Tips

• Supplemental wave kits are also available from Flinn Scientific: Standing Wave Generator, AP6161, Transverse Wave Demonstration, AP6252, Simulated Double-Slit Interference, AP6626, and the Singing Tube Demonstration Kit, AP6312

Further Extensions

Photoelectric Effect Simulation

Wedge a small (approximately 1 cm x 1 cm) piece of thin cardboard or heavy paper between the coils in the middle of the extended wave demonstrator spring. The cardboard piece should be just small enough to fit into the coil so that it will not touch the floor when the wave demonstrator is wiggled. Use the Transverse Wave Procedure to produce a second or third harmonic standing wave (a standing wave with two or three peaks). Observe that the cardboard square remains inside the spring. Increase the amplitude of this wave, but keep the wavelength (and frequency) the same. Does the cardboard piece fly out of the spring? Decrease the amplitude of the wave back to the original level. Increase the frequency (decrease the wavelength) of the standing wave while maintaining the same original amplitude. Does the cardboard fly out? Continue to increase the frequency of the wave, while maintaining the same amplitude, until the cardboard piece flies out of the spring.

This demonstration simulates the photoelectric effect. The photoelectric effect occurs when light of some minimum frequency strikes the surface of a metal and electrons are ejected. Higher frequency electromagnetic (light) waves have higher energy than lower frequency electromagnetic waves. Electrons (simulated by the cardboard square) will not be ejected from an atom unless the light wave that interacts with the electron has enough energy to overcome the internal energy the atom has for the electron. The energy unit of electromagnetic waves, known as a photon, is proportional to the frequency of the electromagnetic wave, but is independent of the amplitude. High-intensity (large amplitude) light waves will not eject an electron from an atom if the photon’s energy is too small to overcome the atom-electron forces. High-intensity light has more photons than low-intensity light, but the protons have the same energy.

Note: This simulation will take some practice, and a bit of trial and error. If the amplitude is too large, the cardboard may fly out because the wire coils spread apart more at the antinode’s (peak) position when the amplitude is large. Start with a very small frequency when making the large amplitudes.

Discussion

What are waves? A wave is a displacement or disturbance (vibration) that moves through a medium or space. A wave involves the movement of energy or a disturbance that changes in magnitude with respect to time at a given location and changes in magnitude from place to place at a given time. Waves can move through materials such as springs, air and water. Transverse waves display characteristic properties of wavelength, amplitude, and frequency. Standing waves display characteristic properties of nodes and antinodes. Figure 4 shows a representation of some of these characteristics. Consult standard physics and physical science texts for more details about wave properties.