Visibility of a cell membrane under microscopic Imaging
The membrane(and associated proteins) isn't actually opaque, it is just "more opaque" or "less translucent" than the cytosol: when you look through the microscope, you are utilizing this contrast to detect the edges.

In the center of the cell you observe, you see the membrane straight-on, and light is only affected by 1-2 layers of membrane. On the edge, you are viewing through the membrane somewhat "sideways" , so there is effectively more "thickness" of membrane. Maybe not an exact replica of the situation, but one that might be easier to visualize, is earth's atmosphere viewed from space. You can see the "blueness" of the atmosphere at the edges, but when you look straight down at the ground you see clouds, oceans, landmasses not tinted as blue.