Multiple choiceQuestion 1 · Recall
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By the end you can describe three experiments that demonstrate two-source interference, explain how each produces coherent waves, and say what is observed.
Every experiment in this lesson uses the same trick. Instead of two independent sources, take one source and split its output into two paths.
Whatever the source does — every flicker, every change of phase — happens to both waves at once. They therefore stay in step: they are coherent.
Then let the two waves overlap and look for the pattern: places where they always reinforce, and places where they always cancel.
A ripple tank has a vibrating bar with two small dippers hanging from it. Both dippers touch the water and move up and down together, so they are coherent sources of circular ripples.
The two sets of circular wavefronts cross. Where a crest from one meets a crest from the other, the water moves strongly; where crest meets trough, it stays almost still.
What you see: a pattern of nodal lines — straight or curved lines radiating out from between the dippers, along which the water hardly moves. Between them are regions of large amplitude. The pattern is fixed in space while the dippers vibrate.
Changing the dipper frequency changes the wavelength, and the spacing of the nodal lines changes with it.
Connect two loudspeakers to the same signal generator, so they emit the same frequency in phase. Walk along a line a few metres away, or move a microphone along it.
The sound alternates between loud and quiet as you move: maxima where the path difference is a whole number of wavelengths, minima where it is an odd number of half wavelengths.
The positions of the loud and quiet spots are fixed, because the sources are coherent. If you drove the speakers from two separate signal generators, the pattern would wander and you would hear no steady alternation.
Two small horn antennas are fed from the same microwave oscillator and point at a detector.
Move the detector across the region where the beams overlap. Its reading rises and falls: maxima at points where the waves arrive in phase, minima where they arrive in antiphase.
This works well because microwaves have wavelengths of a few centimetres, so the maxima are far enough apart to locate with a probe. It is the microwave version of the double-slit experiment.
You cannot simply put two lamps side by side: they are incoherent.
Light also has a wavelength of only about , so the maxima are extremely close together unless the sources are very close and the screen is far away.
The solution, used by Young, is to pass light from one source through two very narrow slits. The slits act as two coherent sources. That experiment is the subject of the next lesson but one.
Two loudspeakers playing the same note always interfere.
They must be driven from the same source so that they are coherent. Two separate players drift out of step and the pattern wanders.
The nodal lines in a ripple tank are places where no waves arrive.
Both waves arrive there; they cancel because they are in antiphase. The water is still, not empty of waves.
Switching off one dipper leaves the same pattern but fainter.
The pattern disappears entirely: with one source there is no interference, just a single set of circular ripples.
Microwaves cannot show interference because they are not light.
For each experiment, be ready to state: the source, how the two waves are made coherent, what is observed, and how the observation depends on wavelength.
"Why are the sources coherent?" is answered by "they come from a single source" — not by "they have the same frequency" alone.
When describing the ripple tank, say that the dippers are attached to the same vibrating bar; for sound, that both loudspeakers are driven by the same signal generator.
Any waves interfere if they are coherent. Microwaves are ideal because their wavelength is convenient for the apparatus.