1.2 Introduction
1.2 Constructive and Destructive Iinterference
1.2 Phase
1.2 Standing Waves
1.2 Summary
Unit 1.1 Multiple Choice Questions - Waves
Unit 1.1 Multiple Choice Answers - Waves

Unit 1.2 Superposition and Interference

Constructive and Destructive interference

Consider a string with two pulses... one approaching from the left and the other from the right as in the following diagram.


two waves

Apart from during the actual encounter it is clear that there are two pulses on the string and further more they pass through each other without any modification. This is true for all types of waves.

But what happens to the medium as they actually try to share the same space and time?

Did you notice at that moment they formed ONE wave of larger amplitude?

This is because the displacement from each wave adds to the each other!

What do you think will happen if one pulses was a crest and the other a trough of equal amplitude?


Did you notice that when the pulses meet they cancel out each other?

So the actual amplitude is the addition of the displacement from each pulse. Recall that displacement is a vector quantity and as such has direction attached to it.

This is called the Principle of Superposition and can be the result of any number of waves.

Interference is the phenomenon of what happens when two waves share space and time.

Constructive interference - the resulting amplitude is larger than either amplitude.

Destructive interference - the resulting amplitude is smaller than either one or both amplitudes.

Interference of Two Water waves.

Consider two disturbances on the surface of a pond as in the diagram below.

/interference patern

Wave fronts spread in a circular fashion outwards from both points. In the diagram crests are the white circles and troughs are in purple.

— When a crest meets another crest or a trough meets another trough we get constructive interference and the crest gets higher or the trough get deeper. These are represented in green in the diagram.

— When a crest meets a trough we get destructive interference and if they are equal in amplitude they cancel out each other.

— This is a two dimensional interference pattern.

So the natural question is what happens at other points?


Concept by Kishore Lal. Programmed by Kishore Lal... Copyright © 2015 Kishore Lal. All rights reserved.