Light, Sound and Waves

Superposition effects as a characteristic of wave motion

Teaching Guidance for 11-14 14-16 PRACTICAL PHYISCS

Given some radiation, how can one tell whether or not it has wave properties? Superposition experiments offer an answer. Radio waves, microwaves, and light are all thought to be waves because they show superposition effects, not because anything can be seen to be oscillating.

Applications of superposition include the use of reflector and director rods in a television aerial; the fading of v.h.f. radio when an aircraft passes overhead and there is superposition of direct and reflected waves; the blooming of lenses to reduce reflections; the acoustical design of concert halls (it is important to avoid creating dead spots where certain frequencies, or notes, are obliterated by destructive interference; good design will also prevent constructive interference which can locally increase the volume of a particular note).

It is important that students have an opportunity to do experiments involving superposition, for example using 1 GHz equipment, producing optical interference fringes and observing 2-slit interference with microwaves. They may also watch teacher demonstrations or hear reports about the other relevant experiments. What they should understand from discussing a whole group of experiments is that wave motions show superposition behaviour. This involves regions where waves are in phase giving high intensity, and regions where waves are in antiphase giving low intensity.

In many practical physics experiments, superposition effects are observed and the wavelength can be found by measuring a path difference. If the frequency is known, the speed of the waves may also be calculated.

Some of the experiments involve reflection. Reflection often, but not always, results in a phase change of p. If there is a node at the reflecting surface, then a phase change of p is occurring. In calculating the wavelength, either this phase change must be taken into account by adding an extra ½λ to the path difference, or the method employed must be adapted so that it will not affect the results: by, for example, making the path difference change by one wavelength, so that the received signal changes from a minimum through a maximum to another minimum.

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