Diffraction of waves
Imagine that some (blue) waves were washing against a (red) barrier. If a bit of the wave went through a hole in the barrier it would be like a spike of water in an otherwise calm place. That would not be stable, the spike would collapse. It collapses outwards in a semi-circle.
For the spreading (diffraction) to be semicircular the gap has to be a similar size to the wavelength.
It is not just water waves that do this but all waves. That is why we can hear sound around the corner of a building or a doorway or over the top of a wall.
Some of the energy of the sound waves diffracts around the top of the wall. However the shorter the waves the less they diffract. The lady can be heard from over the wall (by the way, the man thoroughly deserves to be shouted at) but she cannot be seen. That is because the waves of light are very short and the amount they diffract around the wall cannot be seen.
Light will diffract noticeably through very small gaps, this can be seen in Young's slits experiment or you can try yourself with a tiny pin hole in a sheet of aluminium foil.