Predict the interference pattern

Ilka leans over the two-rod ripple tank. One spot of water catches her eye: ripples stream past on both sides of it, but the spot itself hardly moves.

Big lanes here, still lanes there. What decides which spots swing big and which stay nearly still?

Each spot receives ripples from both rods. From each rod, crests and troughs take turns arriving.

At some spots, the two sets arrive in step: a crest from one rod lands together with a crest from the other.

At this spot, a crest from each rod arrives together: the overlap is constructive every cycle.

Crest with crest is a constructive overlap, repeated every cycle, so the spot swings extra high and extra low.

At other spots, the two sets arrive out of step: a crest from one rod lands together with a trough from the other.

Crest with trough is a destructive overlap, repeated every cycle, so the spot stays nearly still.

At this spot, a crest from one rod arrives with a trough from the other: the overlap is destructive every cycle.

That is the whole prediction. Crest-with-crest spots swing big; crest-with-trough spots stay nearly still.

Half a cycle later, troughs from both rods arrive together along the same lane. The displacements are again on the same side, so these spots also swing big.

At one spot in a two-paddle pond pattern, a crest from each paddle always arrives at the same moment. Predict what the spot does.

Crest arriving with crest is a constructive overlap, repeated every cycle.

The spot swings with bigger ripples than either paddle makes alone.

At another spot, every crest from one paddle arrives together with a trough from the other. Predict what the spot does.

Crest arriving with trough is a destructive overlap, repeated every cycle.

The spot stays nearly still.

At one spot in a two-source ripple pattern, crests from both sources always arrive together. What does the water at that spot do?

At another spot, every crest from one source arrives together with a trough from the other source. What does the water at that spot do?

Lazlo says: 'A spot where a crest from one source always arrives together with a trough from the other stays nearly still.' Is Lazlo correct?

Your turn

Rain drips from two spots on a gutter into a puddle at the same steady rate. At one spot in the puddle's pattern, crests from both drip points always arrive together. What happens at that spot?

Kerstin dips two fingers into a bathtub in the same steady rhythm. At one spot, every crest from one finger arrives together with a trough from the other. What happens at that spot?

In a two-source ripple pattern, one spot stays nearly still while ripples keep arriving from both sources. Which arrival pattern explains this?