Diffraction (EM)

Diffraction is the bending of electromagnetic waves around edges and through apertures, and the spreading of beams that would otherwise propagate as strict geometrical-optics rays. It is a direct consequence of the wave nature of light: every point on a wavefront acts as a source of secondary wavelets (Huygens's principle), and their superposition produces spreading into the geometric shadow whenever a wavefront is truncated by an aperture or obstacle.

The scale of the diffraction effects is set by λ/D, where λ is the wavelength and D is the characteristic aperture size. For visible light (λ ≈ 500 nm) passing through a 1 mm aperture, the angular spread is ~ 0.5 mrad — essentially negligible on a laboratory bench, which is why geometric optics works. Through a 1 μm aperture, the spread is ~ 0.5 rad (~ 30°) — sharply visible. Diffraction sets the fundamental resolution limit of every imaging system: a telescope with aperture D can resolve angular features of about 1.22 λ/D (the Airy-disc criterion); a microscope objective with numerical aperture NA can resolve spatial features of about λ/(2·NA) (the Abbe limit). Bypassing these limits requires non-far-field techniques (near-field microscopy, stimulated-emission-depletion microscopy) or very short wavelengths (electron microscopy, X-ray imaging). Diffraction is the reason telescopes get larger, radio antennas use phased arrays, and EUV lithography at 13.5 nm is the current enabler of semiconductor feature scaling.