Once upon a time, I knew that ‘opaque’ had something to do with whether you can see through something, but I thought it was a synonym of ‘transparent’, not an antonym.
Since most things are not transparent, we don’t use ‘opaque’ nearly as much as ‘transparent’ to describe things; the opaqueness of material objects is assumed by default.
However, ‘opaque’ beautifully describes ideas that are somehow inscrutable—inaccessible because perfectly obstructed by some stark, looming, indifferent, featureless impediment.
If transparency is commonplace and opaqueness is abstract, translucence, the middle child, is enigmatic, shimmering and mystical. It evokes iridescent dragonfly wings and Tiffany windows.