One lamp lights two narrow openings. Beyond them the light overlaps, and the screen shows a row of fine bright and dark bands. The bands appear because the two openings share one wave: where the paths match, the light adds, and where they differ by half a wavelength, it cancels.
A single geometric ray through each hole would land on two separate spots. The bands need each opening to send light across the screen. Move the screen farther away and the bands spread; bring the openings closer and they spread as well.
