Two sources. One interference pattern.
Follow the wavefronts, compare the distances and discover what happens where the waves meet.
TOP VIEW · RIPPLE TANK
t = 0.00 s
S₁ · teal
S₂ · orange
Solid circles = crests
Dashed circles = troughs
Drag P or click anywhere inside the tank.
Darker purple = larger resultant amplitude. Pale regions = smaller amplitude. This map shows amplitude, not instantaneous displacement.
Investigate point A
Count the wavelengths from each source. Is the path difference a whole number of wavelengths or a whole number plus half a wavelength?
Remember: two troughs also produce constructive interference.
DISPLACEMENT AT THE SELECTED POINT
What happens here over time?
S₁ contribution
S₂ contribution · dashed
Resultant = S₁ + S₂
The graph covers two periods. The moving cursor shows the current phase within this repeating window.
Compare the path difference
Δr = |r₂ − r₁|
Constructive: Δr = 0, λ, 2λ, 3λ …
Destructive: Δr = ½λ, 1½λ, 2½λ …
Other path differences give intermediate interference.
Explore the centre line
Place P halfway between the sources, anywhere above them. Compare r₁ and r₂.
Now move P sideways. How does the path difference change?
A constructive point also passes through zero displacement. Watch a full cycle before deciding what happens there.