Nice simple practicle example.

To Comment 2: Scattering may occur at any ray-surface intersection. Clearly bulk scattering requres a volume, but surface scattering can be applied to any object. - Mark

This is fine for introductory level, And very help ful to go furthur. But Here we are specifying the amount light scattered in specular direction. There may be few application in which you may be knowing this scatter function. What about the cases where I knew only the rms surface roughness of the mirror? How to know the amount of light scattered into specular direction. Thanks... Raja

wonderful！
thanks

The knowledge base is excellent. Name a problem, and its solution is described here. Previously I tried to figure it out using the manual. However, in my experience the best way to tackle a problem is to first search in the KB and see the manual when extra details are needed.
One remark on the article. At page two it is written: "The absorption coefficient is computed automatically via A = 1.0 – R – T". It is good to mention that this only holds when R+T<1. For R+T>=1, A=0 and R and T are scaled down to make R+T=1. This is important to know when you need to model a partially absorbing surface.