Moving waves. Stationary lines.
Follow a nodal or antinodal line and discover what stays the same.
TOP VIEW
Solid purple: antinodal lines
Dashed orange: nodal lines
Teal circles: S₁ crests
Blue circles: S₂ crests
AT THE SELECTED POINT
What is the water doing at P?
S₁ contribution
S₂ contribution · dashed
Resultant
θ is measured from the central perpendicular bisector, not from the line joining the sources.
The coloured curves use the exact path difference. Far from the sources they approach straight directions, where Δr ≈ d sin θ.
Move P along one line
Do r₁ and r₂ change? Does their difference change?
Select an antinodal line. Does its water stay at a crest, or move up and down?
Now select a nodal line. What happens to the resultant throughout the whole cycle?