In case of single crystal no question of scattering center comes, hence it is anyway transparent.

In case of polycrystalline sample, instead of thinking about small grain it can be thought of as distance between two scattering centers where the scattering center means grain boundary. In other words, if the distance between the two scattering centers is less than the wavelength of the light then the medium will appear transparent. Clearly, "the distance between two scattering centers" is the area between the two grain boundaries, i.e. grain size.

Hope this clears the issue.

thanks alot, but is this distance will be filled with gas to form pore or can be pore?

Chandra Bhanu Basak

The distance between is nothing but the grain itself. I don't understand where the pore/gas is coming from!!