擋住光
這一支就是核心畫面的字面版本，差別只在你把它當性質、當數字，還是當一塊地方。
noun · C1
The quality of not letting light through, so that you cannot see what is on the other side.
不透明（的性質）
核心畫面
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.
opacity 的核心畫面可以想成「光走到這裡就停住了」。後面的東西被藏起來，不是因為太遠，而是因為什麼都穿不過去。…更多收起
opacity 的核心畫面可以想成「光走到這裡就停住了」。後面的東西被藏起來，不是因為太遠，而是因為什麼都穿不過去。這個字的每一個意思都是這個「停住」的一種版本：被濁水擋住的光、被窗簾擋住而且可以標成數字的光、被眼角膜上一小塊白濁擋住的光，還有被一份沒人肯解釋的規定擋住的理解。看到這個字，先問一句：這裡被擋住的是眼睛，還是腦袋？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
Two branches
擋住理解
把被擋住的東西從光換成理解，畫面完全沒變：你知道後面有東西，就是看不進去。
noun · C1
The quality of being hard to understand, especially when writing, a rule or a decision is not made clear to the people it affects.
難懂；不透明（指資訊）