Darkness is what happens when there is no light. Shadows are darkness that happens when something gets in a way of a light source, (rather than a lack of a source in the first place).
This simple concept has created many tropes in fiction,
although this index is just for tropes that actually related to lack of light, not metaphorical meanings of "dark" and "shadow" (unless a trope involves both the literal and metaphorical meanings).
Compare
Evil Tropes,
Horror Tropes.
Tropes
