Vibrations account for diffraction
Physics Narrative for 14-16
Multiple paths all related: waypoints across one slit
The explanation for diffraction already presented might seem a little vague: we suggested that passing through a slit allows the waves to spread because that is their natural motion, if unconstrained. That's what a point source is. A classical development of this argument, replete with complex diagrams, is Huyghen's construction. But we don't suggest that this is simple, or for students of this age.
Instead, we suggest that you think about this in a thoroughly modern way, going back to the paths, as these allow us to combine possibilities in ways that predict where photons will be detected. The physical situation of radiations passing through a slit can be modelled by considering the contributions of several paths starting on the slit. Of course, we could start the paths earlier, but if it's a beam arriving at the slit, the contributions will be in step as they leave the slit. It's only the differences in trip time that count, not the actual trip time.
Again, here you're only using geometry: the differences in trip time arise because of the different path lengths and the constant speed of propagation. Where the differences in trip time are small, there the contributions will combine so as to predict the arrival of large numbers of photons per second. No surprises: this is the direction in which the beam ploughs straight on. No, the surprises come for off-centre directions, where some photons are detected.