Why is a foggy window opaque?

dipole

Fog condensed on a glass window is just a very thin layer of water droplets, so why is it so opaque to light? Normally light passes through a water-glass interface just fine, but why not in the case of condensation?

Nugatory

It's not just the water-glass interface that's the problem, it's the water-air interface. You have lots of tiny beads of water on the glass, so incoming light will strike perpendicular to the surface of the water at one point (near the center of a droplet) yet almost parallel to it at another very close point (the edge of the droplet).

Take a second piece of glass, press it flat against the first so that water in between is pressed down into auniform thin sheet between the two surfaces, and it will go back to transparent again.