Phong Model or Phong Specular Reflection Model
It is an empirical model that is not based on physics, although physical observation. Phong observed here for extremely shiny surfaces the specular highlight was minute and the intensity fell off quickly, whereas for duller surfaces this was superior and fell off more slowly. He decided to allow the reflected intensity be a function of (cos θ)n with n >= 200 for a shiny surface and n minute for a dull surface. For an ideal reflector n equals infinity, and for a part of cardboard n equals 0 or 1. In the diagram demonstrated below we can observe how the function (cos θ)n behaves for various values of n. This empirical model for computing the specular reflection range was developed via Phong and therefore termed as PHONG MODEL/ PHONG SPECULAR REFLECTION MODEL. That is the intensity of specular reflection is proportional to cosn a (a lies in between 0° & 90°) consequently cos a varies from 1 to 0. Here 'n' is specular reflection parameter dependent upon the type of surface.
Remember that the Phong illumination equation is simply the Lambert illumination equation along with an additional summand to account for ambient reflection and specular reflection.