The answer is complicated and a lot of mathematics is involved.
In free space photons behave like a stream of bullets but when they strike an obstacle photons behave like sine waves and oscillations occur creating harmonics.
Apparently Leonardo Da Vinci in 1490 first observed this diffraction effect of shadows from a straight edge cast onto a floor.
This article here has a good explanation of this phenomena, but a lot of maths is involved and its over my head.
http://serge.bertorello.free.fr/opti...c/diffrac.html