Every part of the gap acts as a source (Huygens), so the wave bends around both edges. Widen the gap and the middle passes almost straight through with only the edges curling; narrow it toward one wavelength and the two edge waves take over, spreading almost in a semicircle.
MissionPredict the gap width that gives the greatest spreading.Streak 0Best 0
Diffraction at a gap is greatest when the gap width is:
gap width120
wavelength40
spreadingsmall
The wave diffracts at the two edges of the gap. A wide gap lets most of the wave pass straight on; as the gap narrows toward the wavelength the edge waves dominate and the spreading becomes almost semicircular.