Destructive Interference - Physics

When two waves collide while traveling through the same medium, what happens? What effect would the waves colliding have on the medium's appearance? Will the two waves collide and bounce off one other or will they travel through each other? Wave interference is a topic that deals with the collision of two or more waves traveling through the same medium.

Definition of Interference

When two waves collide while traveling across the same medium, this is known as wave interference. The medium takes on a shape as a result of the net effect of the two distinct waves on the medium's particles due to wave interference. Consider two pulses of equal amplitude flowing in opposite directions along the same medium to begin our investigation of wave interference. Assume that each wave is 1 unit higher at its crest and has the shape of a sine wave. As the sine pulses become closer to each other, there will come a point where they are fully overlapped.

The resulting shape of the medium at this point is an upward displaced sine pulse with a 2 unit amplitude. The diagrams below show snapshots of the medium before and during interference for two different pulses. Individual sine pulses are depicted in red and blue, with the resulting medium displacement depicted in green.

Constructive Interference

Constructive interference is a type of interference that happens when two interfering waves have the same displacement in the same direction at any point along with the medium.

Destructive Interference

Destructive interference is a type of interference that happens when the two interfering waves have a displacement in the opposite direction at any point along with the medium. Destructive interference occurs when a sine pulse with a maximum displacement of +1 unit meets a sine pulse with a maximum displacement of -1 unit. The graphic below illustrates this.

The interfering pulses in the diagram above have the same maximum displacement but travel in different directions. When the two pulses are totally overlapped, the effect is that they utterly destroy each other. There is no subsequent displacement of the medium's particles at the instant of total overlap. This state of "destruction" is not permanent. In truth, claiming that the two waves annihilate each other is only half accurate. When the two pulses are overlapped, the effect of one of the pulses on the movement of a specific medium particle is annihilated or negated by the influence of the other pulse.

Each particle pulls on its nearest neighbor as waves convey energy through a medium. The upward pull of one pulse is balanced (canceled or annihilated) by the downward pull of the other pulse when two pulses with opposite displacements (i.e., one pulse displaced up and the other down) meet at a particular location. There is still an upward displaced pulse and a downward displaced pulse after the two pulses pass past each other, both going in the same direction as before the interference. Destructive interference only causes a temporary circumstance in which the medium's displacement is less than the largest amplitude wave's displacement.

Because the two interfering pulses have opposite displacements, this is also destructive interference. The interference's destructive character prevents total cancellation in this scenario.

Surprisingly, the collision of two waves along a medium has no effect on the individual waves, nor does it cause them to stray from their intended route. When compared to what happens when two billiard balls collide or two football players collide, this behavior is astounding. Billiard balls may collide and bounce off one another, just as football players may collide and come to a halt. However, two waves will collide, form a net resultant shape of the medium, and then go back to doing what they were doing before the irradiation.