Why Do Square Holes in Bamboo Slats Create Round Light Patterns?

[PatrickPowers]

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?

 

[Jano L.]

What size do these holes have?

 

[PatrickPowers]

What size do these holes have?

Oh, a few millimeters on a side. There's no chromatic aberration.

 

[AlephZero]

This has nothing to do with quantum effects. Each hole is acting like a pinhole camera, and the "round" patch of light is an image of the sun.

 

[chrisbaird]

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.