Ideal Diffuse Reflection ...

...On a ideal diffuse reflector : a very rough surface.

First, we will consider a particular type of surface called an ideal diffuse reflector. An ideal diffuse surface is, at the microscopic level, a very rough surface.
Chalk is a good approximation to an ideal diffuse surface.

Because of the microscopic variations in the surface, an incoming ray of light is equally likely to be reflected in any direction over the hemisphere.