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1. If a diffraction grating is smaller, approaching infinitely smaller, than the wavelength of incident light, what happens to the diffraction pattern? Does the wave still diffract at all?

2.

Wavelength is the distance between two crests of periodic motion. Imagine a wave moving along a horizontal axis that runs into a vertical barrier with a single slit in it. How can the wavelength, a measure of horizontal distance parallel to wave propagation, interact with the length of the slit, a measure of vertical distance? How does the incident crest "know" this ratio of slit length to wavelength in order to diffract accordingly; what tells the incident crest, for example, how far its subsequent crest is away?

If I think of the instantaneous moment a crest meets the slit, and treat that crest as a point source according to the Huygen principle, I don't understand how this point source's subsequent pattern can depend on its distance from the wave behind it. I could freeze the picture at the moment this crest meets the slit, and then adjust its distance to the crest behind it and seemingly arbitrarily alter the outcome of how it diffracts. I don't know of any long-range interaction between water waves, for example, beyond perhaps a force pushing the waves in the direction of propagation -- nothing that would tell the foremost crest how it should diffract when it hits a slit of arbitrary separation.

As an analogy, if I were to view the crests as particles that diffract upon striking the slit, it makes no sense to me that a particle meeting the slit would split up depending on its distance to the particle behind it. What property of this secondary particle is interacting over the distance with the incident particle in order to influence how it will diffract?

In the case of water, it would make more sense to me if the wavelength had some component along the ripple, as that is in the same dimension as the slit itself, rather than an orthogonal component -- for example, if the separation of waves were parallel to the wave propagation.

Can someone provide conceptual insight to how this works? Thank you very much!