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Physics Lesson 11.4.1 - What is Interference?
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Welcome to our Physics lesson on What is Interference?, this is the first lesson of our suite of physics lessons covering the topic of Interference of Waves, you can find links to the other lessons within this tutorial and access additional physics learning resources below this lesson.
What is Interference?
By definition, interference is the combination of two or more waveforms to form a resultant wave in which the displacement is either reinforced or cancelled.
Look at the figure.
The resultant wave Z is obtained by the overlapping of waves X and Y. Therefore, we can say that interference is the process of the overlapping of two or more waves.
Interference is produced as a result of the principle of superposition, a key principle in waves theory, which says:
The waves pass through each other without being disturbed. The net displacement of the medium at any point in space or time, is simply the sum of the individual wave displacements.
The principle of superposition may be applied to waves whenever two (or more) waves are travelling through the same medium at the same time.
There are two conditions the waves must meet to produce interference patterns.
- Waves must be of the same kind. Obviously, a light wave cannot interfere with a sound wave as they are of different nature. Light is an EM wave and it can spread only in transparent media, regardless whether they are mechanical or not, while sound waves are mechanical waves that can spread only in material media. Furthermore, the method of propagation is also different; sound waves propagate in a longitudinal way while light is a transverse wave. Hence, it is impossible for these two kind of waves to overlap.
- Waves must have the same frequency in order to produce a standing interference. Standing waves have features that do not change with time. If it was not so, waves could interfere at one point but not in the others. Therefore, the effect of interference would be not visible as the shape of the resulting wave would be irregular.
The above case occurs when we hear two people talking simultaneously, or when waves of a radio channel interfere to another, so we cannot hear well.
In this regard, we can say that it is wavelength (and therefore, frequency) the quantity that determines the interference pattern. This means only like waves with the same frequency (wavelength) can produce interference in certain conditions. This last phrase, i.e. "in certain conditions" represents another restriction for the waves to produce interference. This condition is explained in the next paragraph.
You have reached the end of Physics lesson 11.4.1 What is Interference?. There are 3 lessons in this physics tutorial covering Interference of Waves, you can access all the lessons from this tutorial below.
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- Continuing learning waves - read our next physics tutorial: Sound Waves. Intensity and Sound Level
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