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
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.