The house where I live has a wall made of bamboo slats. There are small square holes between the slats, but when light shines through the square holes onto a surface the shape is always round. Why is that?
It depends how big the holes are and how far away the imaging surface is. It could be a pinhole camera effect or it could be simple diffraction if the distance is far enough, both of which are classical wave effects. For instance, a water wave in the ocean flowing passed a square obstacle will have a square hole in its wavefront at first, but that will quickly disappear.