ok its simple
when water waves pass thru a gap, if the gap's length is equal to the wave's wavelength then there will b a spherical diffraction,
but if the wavelength of the wave is lower than the gap of the length then the waves formed will hav defraction on their extreme edges, they will look like straight lines that are bent on the edges.
we don't hav the diffraction of waves such that the length of the gap is smaller than the length of the wavelength.
remember the top two points all the best