Fresnel effect
Surfaces reflect more light when you look at them from a low, grazing angle.
See it in WebGL playground. Set Shader to Fresnel glow: the edges of the shape glow where you see it side-on.
In plain words
Stand at a lake and look straight down: you can see the bottom. Look far across it, and the water reflects the sky like a mirror. The same surface reflects more when the angle is low. This is the Fresnel effect, named after a French scientist. 3D tools add it so edges of glass, water and plastic look right.