Optics: lenses, focus and aberrations

Die Beugung is the optician’s word for diffraction: light spreading out behind a narrow opening or bending round an edge, which is why stopping a lens down too far makes the picture softer rather than sharper. Most learners have met the word long before, in a grammar book, where die Beugung is inflection — the endings a noun, an adjective or a verb takes. Both senses come from beugen, to bend: a word is bent into its forms, a wave is bent at an edge. The same verb gives das Knie beugen, to bend the knee, and sich einer Mehrheit beugen, to bow to a majority. In court it turns darker: die Rechtsbeugung is the offence of a judge or an official bending the law. So when die Beugung turns up, its neighbours decide the meaning. A slit, an aperture or a wavelength means physics; a case, an ending or a verb means grammar.