When black and white photo film is completely overexposed and developed, will it be completely black opaque (you can't see light through) or will it be black but not opaque (you can see light through)? and why is this so?
Fully exposed (i.e. sitting out in room light to the limit of the H/D curve) B&W film that's fully developed is not completely opaque. Many years ago, this was recommended as a filter for viewing the sun's disk and solar eclipses (there are better, safer filters now, please don't try this) -- because you could see the sun through the film.
This is film at the highest density obtainable. Still not completely opaque.