現實中有咩不是光學介質(optical medium)?

From your given link: https://en.wikipedia.org/wiki/Optical_medium , an "optical medium" is a substance through which electromagnetic waves propagate.

Following this definition, metals (including alloys) are not optical media, as metals are conductors and which electromagnetic waves cannot pass through.
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Reply to your question in "意見":

You are asking, in your question, the material substances that are not optical medium. Copper, clearly, is NOT an optical medium, as electromagnetic waves cannot pass through (see the definition of optical medium in your second link).

The reflection of light on polished copper surface indicates that light wave cannot pass through the substance.

(1) 光介質 --- 可傳輸光線的質料(a material which allows light to transmit throught it) eg. 空氣,水,玻璃,鑽石
★ 入射的光線，會穿過質料，以相同/相近的方向 折射 出來
傳統上、光學介質 限定為：晶態（光學晶體）、非晶態（光學玻璃）、有機化合物（光學塑料）。

(2) 非光學介質 --- 不可傳輸光線的質料 eg. 木材(及其製成品)、磚石(未經打磨/拋光)、粉末(如 碳粉、生石灰、麵粉)
★ 入射的光線，並不可穿過質料，光線會: i)改變方向、反射或散射* 出來；ii) 還會經過吸收，改變光線的強度和光譜成分。

* 光是直綫傳播，a) 當遇到一個「光滑物體」時，會被反射回來，就像乒乓球碰到牆壁被彈回來一樣 --- 稱為 光的「反射」；
b) 當遇到一個表面不「光滑」的物體，入射光雖為平行光束，反射光卻四向分散 --- 稱為 「散射/漫射」
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Your Q: 木材的表面雖不光滑, 但也反射了部分光, 所以人才看到木色吧
Ans: 木材是 非光學介質，不可傳輸光線 ; 但木凹凸不平的表面，可把入射光散射，人就憑 「散射的光」 看到木色!