Why does diffraction produce a pattern of light and dark coloured bands? Or, why doesn’t a normal plane/spherical wavefront give a pattern? I know that diffraction is the changing of a plane wavefront into a spherical wavefront and I understand why it happens (Huygen’s principle), but I don’t get why it gives a pattern of light and dark coloured bands. I don’t get why a diffracted spherical wavefront interferes with itself to give light and dark bands but a normal spherical wavefront, say from a concave lens, or a plane wavefront, doesn’t give them.

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*Take a plane wavefront ||| for example, the light from the bottom of a | is out of phase with the light from the top of the |, but their interference doesn’t give a pattern.

*Say now that I pass ||| through a slit, the the light from the top of | interferes with the light from below to give a pattern.

Why is there a pattern in the second case and not in the first case ? I can’t visualise it and really understand the interference.
 A: if I am understanding correctly, by out of phase you mean the light that reaches one point from the other is out of phase? Because by definition, a wavefront are contours where a wave has a constant phase.
In the first case I think you may be forgetting the plane is infinitely long. At a point on the wavefront, there are an infinite number of sources superposing in pairs of opposite phases stretching out to +/- infinity. Therefore their contribution cancels and you are just left with the original source point on that wavefront, therefore, there is no difference.
The second case has this diffraction effect because the plane is NOT infinitely long due to the aperture cutting off the edges.
Now, a few extra notes that may or may not improve your overall understanding of waves.
A diffracted wave through a slit does NOT produce a spherical wave, it curves the edges but it does not produce a spherical wave unless the slit is infinitely thin as according to Fraunhofer diffraction.
Think of a point source spherical plane wave - the wavefronts are smooth and there is no diffraction therefore the diffraction cannot be due to the curved gemoetry of the wave.
I hope this helps.
All the best :)
A: Diffraction does not always cause bands, example water waves in a single slit diffract uniformly. Interference is a separate phenomenon, obvious with water waves and 2 slits ... but not so obvious for light ... which shows a pattern for both single and double slits. Light per Feynman chooses paths based on the EM field.
In the dark areas for the DSE there are no photons, they are all in the bright areas.
