Wave Interference

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Wave Interference

Introduction

To understand the concept of interference, let’s refresh our understanding of waves. Waves are disturbances that carry energy through a medium or space. They can be categorized into two main types: transverse waves and longitudinal waves. Transverse waves like ripples on a water surface, move perpendicular to the direction of the wave. Longitudinal waves, on the other hand, move parallel to the direction of the wave e.g. sound waves. Interference occurs when two or more waves meet or overlap. In this article, we will explore the concept of interference, types of interference, and standing waves.

Interference

We all know the fact that all matter has volume. The reason is that objects cannot occupy the same space at the same time. Because waves carry energy but not matter, more than one wave can exist in the same place simultaneously. Two waves can meet, share the same space, and pass through each other without obstruction. When two or more waves share the same space, they interact with each other. This phenomenon is known as interference.

Types of Interference

The result of interference can be constructive or destructive depending on how the waves interact.

Constructive Interference

Constructive interference occurs when the crests of one wave align with the crests of other waves, while their troughs also overlap. An example of constructive interference is shown in the mentioned figure. When waves combine in such a manner, a new wave is formed with higher crests and deeper troughs than the original wave. In simpler terms, the resulting wave has a larger amplitude than the original waves had.

Destructive Interference

Destructive interference arises when the crests of one wave coincide with the troughs of another wave. Consequently, the resulting wave exhibits a reduced amplitude compared to the original waves.

Standing Waves

Recalling the previous experiment, take a piece of rope and tie one end of the rope to the fixed point. Now, move the other end up in and downward direction, and the waves arise travel down the rope, and reflected back. When the rope is moved at specific frequencies, the rope appears to vibrate in loops. The loops arise in the rope due to the interference between the wave created and the reflected wave. The resulting wave is known as a standing wave.

A standing wave is characterized by a stationary pattern, in which portions of the wave are at the rest position due to total destructive interference and other segments have a large amplitude due to constructive interference.

It is important to note that although the wave may appear motionless, in reality, the waves are traveling in both directions. Standing waves can manifest in various types of waves, including transverse waves as shown here, as well as longitudinal waves.

Resonant Frequency

Standing waves can be created at more than one frequency. The frequencies at which standing waves occurred are called resonant frequencies. Resonance takes place when an object vibrating at or close to the resonant frequency of another object, induces vibrations in the second object. As a result, the resonating object absorbs energy from the vibrating object and in turn, begins to vibrate as well.

The Tacoma Narrows Bridge

The phenomenon of resonance played a significant role in the unfortunate demise of the Tacoma Narrows Bridge located in Washington. After its opening in July 1940, the bridge acquired the nickname "Galloping Gertie" due to its noticeable wavelike oscillations.

These oscillations were primarily induced by wind passing over the bridge's structure. The wind caused vibrations that were close to the resonant frequency of the bridge. The wind's vibrations closely matched the resonant frequency of the bridge. Because the bridge was in resonance, it absorbed a substantial amount of energy from the wind and exhibited significant vibrations with a large amplitude.

On November 7, 1940, a supporting cable slipped, leading to the initiation of twisting motions in the bridge. The combination of twisting and strong winds further amplified the bridge's amplitude of motion. Within a matter of hours, the amplitude grew to such an extent that the bridge eventually collapsed. Fortunately, all the people on the bridge at that time managed to escape before it collapsed into the river below.

Summary

When two or more waves interact with each other, they overlap. This phenomenon is known as interference.
Constructive interference occurs when two waves overlap in such a way that they generate a wave with a larger amplitude.
Destructive interference occurs when two waves overlap in such a manner that they cancel out each other.
A standing wave refers to a stationary pattern that emerges from the superposition of two or more waves traveling in opposite directions along the same line.

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