11.3 Superposition

When two billiard balls collide on a pool table they do just that-they collide. They exert forces on each other and change directions. This is aptly described by the old adage that "two objects can not occupy the same space at the same time." However, waves behave quite differently. Waves can pass through each other and continue on; this is known as superposition. When two waves pass through each other the result is just the sum of the two individual waves Figure 11.8 illustrates this for waves on a rope.

Figure 11.8 Waves can pass through each other and continue on. This is known as superposition.

Superposition is a general characteristic of all waves; it is not limited to waves on a rope. Figures 11.9, 11.10, and 11.11 illustrate this idea for other waves.

Figure 11.9 Light from two spot lights passes through undeviated. Light is never bent by other light pushing on it from the side.

Figure 11.10 You can simultaneously hear sound from two different sources. One wave never blocks out another. This ability of waves to pass through one another is known as superposition.

Figure 11.11 Ripples on a lake's surface pass through each other unaffected. This is another example of superposition.

Superposition may also be referred to as interference. Figure 11.12 shows water waves generated on a ripple tank in a Physics demonstration. In some regions the two waves are "in phase"-both trying to create a crest or both trying to create a valley. There we see a wave of large amplitude and call that a region of constructive interference. In other regions the two waves are "out of phase"-one trying to create a crest while the other is trying to create a valley or vice versa. There we see a wave of small amplitude and call that a region of destructive interference.

Figure 11.12 Superposition of waves in a laboratory ripple tank.