Diffraction

Diffraction is the bending and spreading of a wave as it encounters an obstacle or passes through an opening, rather than continuing in a strict straight line as simple ray-based intuition would predict. Every point on a wavefront acts as a source of new tiny wavelets (Huygens' principle), and diffraction is what happens when an edge or aperture selectively blocks some of those wavelets, letting the surviving ones interfere and spread into the geometric shadow region a straight-line model says should be dark.

The effect scales with the ratio of wavelength to obstacle or gap size: diffraction is dramatic when the two are comparable, and negligible when the wavelength is far smaller than the opening. This is why a doorway noticeably bends sound around its edges — audible wavelengths run from meters down to centimeters, similar in scale to a doorway — but barely diffracts visible light, whose wavelengths are a few hundred nanometers, millions of times smaller than the same doorway. It is also why bass frequencies "carry" around corners and through walls far better than treble: a long bass wavelength diffracts around obstacles a short treble wavelength simply reflects off.

The classic demonstration is single-slit diffraction: light through a narrow slit produces not a sharp-edged bright band but a spread pattern of alternating bright and dark fringes, direct evidence that light is behaving as a wave rather than a stream of particles traveling in straight lines. The same physics sets a hard floor on resolution: any lens or telescope aperture diffracts the light passing through it, which is why there is a diffraction limit on how finely any optical instrument can resolve detail no matter how perfectly its glass is ground — the limit comes from the wave nature of light itself, not manufacturing tolerance.