Think of light arriving at something and stopping there. Whatever is behind it stays hidden — not because it is far away, but because nothing is getting through. Every use of opacity is a version of that stop: light stopped by muddy water, light stopped by a curtain you can put a number on, light stopped by one cloudy patch on an eye, and understanding stopped by a rule nobody will explain.
From the word’s history
By parts, opaque plus -ity; through French opacité from Latin opācitās, on opācus, shaded or shut off from light. The root idea is already a thing standing in the way of light.
依證據裡的詞源，opacity 是 opaque 加上 -ity，經法文 opacité、拉丁文 opācitās，最後落在 opācus（有陰影的、擋住光的）。字根本身就是「擋光」的畫面，所以核心意象不用另外借來。
Wiktionary entry for opacity (source_ref kaikki:3f6f69a190036ae4:1334893142), etymology_text supplied in the evidence packet
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