Where two waves meet

Drop two stones in still water and the ripples pass straight through each other — but where a crest lands on a crest the water heaves twice as high, and where a crest lands on a trough it goes flat. Those quiet lines fanning out are the reason we know light is a wave. Drag either source.

"Quiet lines" counts the directions where the two waves cancel out completely. Move the sources further apart and more of them appear — that count is a real prediction you can check against the picture.