When waves reach a narrow slit, the water in the slit vibrates
like a point source. The waves thus sent out from secondary sources
along the slit are nearly in-phase when arriving any point in the
forward direction. The diffracted wave resembles a circular wave
with centre at the slit.
A wave spreads out (noticeable diffraction) when the size of the slit is comparable to or smaller
than the wavelength.
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