Opacity expression based on gradient or noise layer

I am sure this should be a simple one but just can't find the right one that works. I want the opacity of a shape layer to be controlled by the white values on a gradient or noise layer. So I need to sample the area the shape layer travels over and vary opacity acordingly (ie when it travels over a full white area the opacity is set to 100 and when it travels over a black area its opacity is 0).