W AVE I NTERFERENCE

What happens when two waves meet while they travel through the same medium? What affect will the meeting of the waves have upon the appearance of the medium? Will the two waves bounce off each other upon meeting (much like two billiard balls would) or will the two waves pass through each other?

Wave interference is the phenomenon that occurs when two waves meet while traveling along the same medium.

The interference of waves causes the medium to take on a shape that results from the net effect of the two individual waves upon the particles of the medium.

Consider two pulses of the same amplitude traveling in different directions along the same medium. Let's suppose that each displaced upward 1 unit at its crest. As the pulses move towards each other, there will eventually be a moment in time when they are completely overlapped. At that moment, the resulting shape of the medium would be an upward displaced pulse with an amplitude of 2 units.

Constructive interference is a type of interference that occurs at any location along the medium where the two interfering waves have a displacement in the same direction. In this case, both waves have an upward displacement; consequently, the medium has an upward displacement that is greater than the displacement of the two interfering pulses.

Constructive interference is observed at any location where the two interfering waves are displaced upward. But it is also observed when both interfering waves are displaced downward.

I NTERFERENCE Destructive interference is a type of interference that occurs at any location along the medium where the two interfering waves have a displacement in the opposite direction.

At the instant of complete overlap, there is no resulting displacement of the particles of the medium. This "destruction" is not a permanent condition. In fact, to say that the two waves destroy each other can be partially misleading. When it is said that the two pulses destroy each other, what is meant is that when overlapped, the affect of one of the pulses on the displacement of a given particle of the medium is destroyed or cancelled by the affect of the other pulse.

Once the two pulses pass through each other, there is still an upward displaced pulse and a downward displaced pulse heading in the same direction that they were heading before the interference.

The two interfering waves do not need to have equal amplitudes in opposite directions for destructive interference to occur. For example, a pulse with a maximum displacement of +1 unit could meet a pulse with a maximum displacement of -2 units. The resulting displacement of the medium during complete overlap is -1 unit. In this case, the destructive nature of the interference does not lead to complete cancellation.

The meeting of two waves along a medium does not alter the individual waves or even deviate them from their path.

The task of determining the shape of the resultant demands that the principle of superposition is applied. Principle of Superposition: When two waves interfere, the resulting displacement of the medium at any location is the algebraic sum of the displacements of the individual waves at that same location.

Displacement of Pulse 1 Displacement of Pulse 2 = Resulting Displacement +1 =+2 =-2 +1=0 +1-2=

E XAMPLES 1. Several positions along the medium are labelled with a letter. Categorize each labelled position along the medium as being a position where either constructive or destructive interference occurs.

Constructive Interference: G, J, M and N Destructive Interference: H, I, K, L, and O

2. Twin water bugs Jimminy and Johnny are both creating a series of circular waves by jiggling their legs in the water. The waves undergo interference and create the pattern represented in the diagram. The thick lines in the diagram represent wave crests and the thin lines represent wave troughs. Several of positions in the water are labeled with a letter. Categorize each labeled position as being a position where either constructive or destructive interference occurs

Constructive Interference: A and B Destructive Interference: C, D, E, and F