8.2.1—The meaning of the term diffraction

Diffraction is the bending or spreading of waves around an obstacle or through an opening. It is most noticeable when aperture size is comparable to wavelength.

Compare aperture width with λ: narrower gaps produce broader spreading, while much larger gaps give weaker edge effects.

Water waves spread strongly through a gap about one wavelength wide but remain nearly straight through a gap many wavelengths wide.

Diffraction is not refraction: no change of medium is required, and the wave speed need not change.