Understanding Wave Motion: Transverse and Longitudinal Waves
Understanding Wave Motion
Most information reaches us through waves. Sound reaches our ears, light our eyes, and electromagnetic signals our radios and TVs through oscillatory motion. Wave motion transfers energy from a source to a receiver without transferring matter between them.
Imagine raising and lowering one end of a rope. A rhythmic disturbance travels along it. Each particle of the rope moves up and down, while the disturbance travels the length of the rope. The medium returns to its initial state after the disturbance passes. The disturbance propagates, not the medium itself. If you drop a stone into a still pond, waves travel outward, forming ever larger circles whose centers are at the source of the disturbance. The waves carry energy, as evidenced by water splashing on previously dry land when they reach the shore.
If the waves encounter impassable barriers, the water returns to the pool, and things revert to the beginning. The surface water will have been disturbed, but the water has not gone anywhere. The medium returns to its initial state after the disturbance passes. What is transported is a disturbance in a medium, not the medium itself. Consider a field with grass growing on a windy day. Waves travel through the grass. Individual stems of grass do not leave their positions; they sway. The grass at the edge of the road, touching your legs, resembles water coming to the edge in the previous example. During continuous wave motion, grass ranges, “vibrating” between defined limits, but not going anywhere. When the wave motion ceases, the grass returns to its original position.
Wave Speed
The speed of periodic wave motion is related to the frequency and wavelength. If we observe a stationary point on the water’s surface and watch the waves pass, we can measure the time between the arrival of one crest and the next (the period) and the distance between two peaks (the wavelength). Speed is defined as distance divided by time. In this case, distance is wavelength, and time is the period.
Transverse Waves
In transverse waves, the cord movement (up and down) is at right angles to the direction of the wave’s speed. This perpendicular, or sideways, motion is called transverse motion. If you move the cord up and down periodically and continuously, the series of pulses triggers a wave. Because the environmental movement is transverse with respect to the direction in which the wave travels, these waves are called transverse waves. Examples include waves on the taut strings of musical instruments and liquid surfaces.
Longitudinal Waves
Sometimes, the particles of a medium move back and forth in the same direction the wave travels. This movement is along the direction of the wave, not at right angles to it, producing a longitudinal wave. You can demonstrate transverse and longitudinal waves with a slinky or a long, flexible spring. A transverse wave is formed by moving the end of the slinky up and down or from side to side. A longitudinal wave is formed by quickly pulling and pushing the end of the slinky toward or away from you. The medium vibrates in a direction parallel to the transfer of energy. A portion of the spring is compressed, and a wave travels through this compression. Between successive compressions is a stretched region called thinning, extension, or rarefaction. Compressions and rarefactions travel in the same direction along the spring. Sound waves are longitudinal.