[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.

[Enthalpy]

"Opaque" has no absolute meaning.

Aluminium is essentially opaque with less than 100nm. Silicon is a bit translucent at 1µm. Glass get opaque around 0.5m, when you look through the width of a window. The Ocean gets dark around -20m.

It also depends on what attenuation you accept or desire and whether the wavelength fits the material. Submarines are said to be detectable by optical apparatus to -30m or -50m, making the Barents, Baltic, Azov seas and the English Channel deadly traps for them.