[shvcko99]

Does any opaque material already block all sunlight regardless of how thick they are? I mean, does an opaque sheet of 1cm block more sunlight than the same sheet of thickness 1mm?

[Borek]

As a first approximation you may want to read about Beer–Lambert law, which is mathematical (and experimentally confirmed) way of dealing with such situations.

At the same time once the intensity of the passing light gets below some threshold it no longer matters, at some level "light" stops to be continuous, you can't have half a photon coming through.