the.com/brightener

means An additive or substance that makes something look lighter, whiter, or more vivid than its natural state — often by trickery of light rather than true cleaning.

from A plain agent-noun built from 'bright' plus '-ener,' the same pattern as 'softener' or 'sweetener' — words for things that do the verb to something else. 'Bright' itself traces back through Old English 'beorht' to an ancient root meaning to shine or gleam. The word 'brightener' became especially useful once chemists discovered optical brightening agents in the 20th century: dyes that don't remove dirt but absorb invisible UV light and re-emit it as blue, tricking the eye into seeing 'whiter than white.'