Opacity of Objects across layers

I would like to make an object in a given layer opaque for all objects in the lower layers. I am not sure how to do it. When I make an object opaque, all objects below it in other lower layers are showing through. Let me know if there is a way to do it. Thanks.

It is due to my misunderstanding of the order of layers. I thought the one near the canvas in the sidebar layers is the bottom-most layer. It is the other way in fact. Once I changed the order of layers, I got what I wanted. So all is well.