Fresnel effect (named after a french optical scientist that invented lighthouse lenses) is visible in everyday life. It's also easy to spot in 3D if it has been engage. 

Think of standing outside and looking into a car window on a sunny day. the more directly perpendicular you are to the car window, the easier it is to see in. The more the window angles away from you if you move, the more difficult it is to see in. This is because as the angle of the viewer increases, the surface begins reflecting more light.

So, in on round, curving or angled surfaces, the surface will reflect more and simultaneously appear darker or lighter depending on the environment around it. The reflected environment is more intense at the edges of the glass too.

In this example, you can see the wood getting darker towards the edge of the glass while the center of the glass shows it clearly. 

Mark