Hi there!
You may want to consider a different starting point with your Material.
As opposed to choose "Clear Plastic" from the library, look for "Hard Rough Plastic" (Solid) for example.
To answer your question technically, the "Specular Transmission" is responsible for the "See Through" effect.
The specular rays can "Reflect" (Bounce off) a surface, or "Penetrate" (Go through) a surface using the "Transmission" amount.
Note that you can also tint the rays as they go through by multiplying some color in there as shown in the Turquoise render.
But if you did not plan on having a "Clear" effect, the "Hard Plastic Rough" template will give the desired result right out of the box.
Hope this helps!
-Berube
www.johnberube.com
