Two source interference - variable separation - bottom

This short video is useful to talk about two-slit diffraction. The two "sources" of waves, in red, are at the bottom edge of the screen. As we progress through the video, the sources move further and further apart. As this happens, you will notice more and more areas of interference. The dark areas, where the circles "line up" are areas of constructive interference - crest meets crest and trough meets trough. The lighter areas between them, are areas of destructive interference, where the crest of one source wave meets the trough of the other source wave and they would cancel. The "light" and "dark" areas are often called "fringes".