Specular reflection is where the light comes in and reflects out from the surface at the same angle (Fig.06).
In reality both type of reflection will coexist on most objects, varying on intensity from one type of surface to another (Fig.07).
The shader will use the information contained in the Diffuse texture to define how the light that hits an object is partially absorbed by its surface and also diffusely reflected. And the Specular texture will define how the specular reflection will happen (Fig.08).
But, if both Diffuse and Specular are types of reflection that coexist on any surface, why aren't they only one, and why may they have different colors? As in this case where we've got a chess pawn with a brownish (wood) diffuse reflection and a white specular reflection (Fig.09).
At this point important information needs to be added to this equation. Physical/chemical characteristics of a material will affect how the light behaves when hitting its surface. Depending on the material's nature, the specular reflection may remain true to the original color of the light, or may be tinted by the color of the object (Fig.10).
So, in this example, the specular reflection on the toy and the chess piece maintains the original color of the light (white), while the specular reflection on the door knob or the Christmas tree ball is tinted by the objects' colors.
How so? For the purpose of our needs, we may reduce the materials to two basic different types: Dielectrics and Conductors. Dielectric materials do not conduct electricity. Conductors do. So, in a very non-scientific way that should only be used by artists and not someone writing a science paper, we could say that Conductors are all the metals or metallic looking surfaces, and Dielectrics are all the non-metallic surfaces, such as wood, plastic, rubber, clay, fabrics, etc (Fig.11).
And what is interesting for us to know is that Conductors alter the color of the specular reflection, tinting them. While the Dielectric materials will be neutral to the light and not affect the color of the specular reflection.
In the case of the Christmas tree ball, while it is made of plastic it still has a metallic look and that's because it was painted with a metallic finish. So it's made to look like metal. The same way if something made of metal is painted with a truly opaque paint, they will reflect as dielectric, and not as conductor, because the external layer that is reflecting the light is not of a conductive nature (Fig.12).
Please note: There is some very interesting information on the web showing how the electrons of conductor or dielectric surfaces are aligned in specific ways and how it affects the light that bounces on them. But for the solely artistic purpose of this study, knowing that it happens, and not all the details of how it happens, is enough.
So, if you want to create a material in 3D that truly recreates the physical aspects of an existing material, you have to understand its nature and how it affects the light.
Ok, we understood how real world materials behave, but how you replicate that in 3D?
In order to do that we must get one bit of technical information. It is very important for us to know that, during the rendering, diffuse and specular will be calculated on two separate moments. First the diffuse reflection will be calculated, then the specular reflection. Then, right after that, one is added to the other for the final result and the object is finally drawn on screen (Fig.13).