An interesting use for the Opacity value is for creating AGS (stands for Architectural Glass Solution). An AGS is a "sort" of glass material, very useful for window panes on architectural projects, where you don't need the time consuming properties of actual dielectric glass (refraction and caustics) but you want transparency and reflection.
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Creating a AGS fast rendering glass by reducing the Opacity of a mirror material. You can download this material example here.
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