瞬

Left eye radical — the indexing element, a pictograph of an eyeball with its inner detail. Puts 瞬 in the eye-action family with 看 (look), 睡 (sleep), 眨 (wink), 瞧 (peek), 瞪 (stare). The radical states the obvious: a wink or blink is something the eye does, and from that micro-action grew the meaning 'an instant, the blink of an eye.'

Right 舜 supplies the sound shùn with no drift. 舜 is mostly known as the name of the legendary sage-emperor; it also names a day-flowering hibiscus whose bloom opens and closes within a single day — a fitting semantic echo for 'in the blink of an eye.' The phonetic carries both the reading and the fleeting-flower image.