It is probably a very stupid question but why do have many shaders (diffuse, glossy, ...) a normal input ? What happens with that normal input ?
Maybe it helps when I clarify the background of my question...I am trying to understand the concepts of Physically-Based Shading and the implementation of these concepts in Blender. See also my questions here and here.
After some discussion with @piegames I noticed that I do not really understand why the normal input is used in shaders. I think I understand what a normal is but with the concept illustrated below I have difficulty to understand what Cycles does with this normal input when calculating the behavior of a shader