Q.What is diffraction of light? How does it differ from interference? What are Fraunhoffer and Fresnel diffractions?

[!TLDR] Diffraction is light bending into an obstacle's geometric shadow, noticeable when the obstacle/aperture size is comparable to the wavelength; it differs from interference (two/few sources) by involving very many overlapping wavelets, and splits into Fraunhofer (plane wavefronts, effectively infinite distances) vs. Fresnel (curved wavefronts, small distances). [!ANSWER] Diffraction = bending of light into shadow regions (via Huygens' secondary sources at an obstacle's edge), noticeable when obstacle siz …

Ordinary ray (geometrical) optics predicts that an opaque obstacle should cast a perfectly sharp-edged shadow, with light travelling in strictly straight lines right up to the obstacle's boundary. In reality, under the right conditions, light is observed to bend somewhat AROUND an obstacle's edges, reaching slightly into the region that ray optics would predict should be in total shadow -- this bending is DIFFRACTION, a phenomenon exhibited by every kind of wave (it is easily heard for sound, e.g. hearing someone talking from around a corner, because sound's much longer wavelength makes its diffraction far more obvious than light's). Physically, diffraction happens because points right along the obstacle's EDGE act, per Huygens' principle, as secondary sources emitting wavelets into the shadow region exactly as any other unobstructed point on the wavefront would -- light does not literally 'curve', it is simply that edge-located secondary sources are not fully cancelled out the way they would be by neighbouring wavelets on an unobstructed wavefront. Diffraction becomes noticeable specifically when the size of the obstacle or aperture is comparable to the wavelength of the light (or wave) involved -- for typical everyday obstacles, vastly larger than light's ~500 nm wavelength, diffraction effects are present but far too small to notice, whereas for a narrow slit only a fraction of a millimetre wide, they become clearly visible.

How diffraction differs from INTERFERENCE: interference, as studied earlier in this chapter, is the combination of a SMALL number of distinct coherent sources -- typically just two, as in Young's double slit. Diffraction, by contrast, can be understood as the interference of a very LARGE number of secondary wavelets, all originating continuously across the width of a single slit or aperture -- so diffraction is, in a real sense, a more general and more elaborate case of the same underlying interference principle, rather than a wholly separate phenomenon. …

Define diffraction via Huygens' principle at an obstacle's edge, state the size condition (obstacle ~ wavelength) for it to be noticeable, contrast it against interference by number of interfering sources (two vs. very many), and distinguish Fraunhofer ( …

Describing diffraction and interference as two completely unrelated phenomena, rather than recognising diffraction as itself a form of interferen …