I have a question regarding light bundles and the diffraction of waves. I've been trying to wrap my head around the processes that govern how diffraction works and it all seems to make sense to me regarding water waves and sound. If I just apply Huygens' principle that every point in a wave is to be considered a point source for a similar wave, the behavior in a ripple tank can be explained: an advancing wavefront, which is actually a row of point sources reinforcing each other, reaches a wall with a slit in it; part of the wavefront goes through a short tunnel, where the point sources still reinforce each other because they form a row as wide as the tunnel; then at the end of the tunnel, the wavefront is out in the open again, where the two outer borders of the row can't be reinforced from the side because that's where the row ends, so the wave diffracts in 180 degrees. If I apply the same logic to light waves I can understand the going-through-the-tunnel part and the emerging-at-the-other-end part, but I don't understand how light behaves BEFORE it goes through the slit. The problem with light is that it moves in bundles, which have borders similar to the ones of the wavefronts that emerge at the other end of a diffracting slit, so Huygens' principle would suggest that light in a bundle would diverge to all possible sides too, which it clearly doesn't. So my question is: what is wrong with my understanding of the concept?