I apologize if this is not in the correct section of the forum, but it seems most fitting. In the context of dielectric materials, I am having trouble understanding why in the discussion of the optical properties of magnetized bodies, it is justifiable to assume for light vibrations, permeability = 1. I have read several texts stating that this is an assumption but I do not understand the physical basis of making such an assumption. One texts simply states, "The reason for this is not that the magnetization of a body cannot follow the rapid changes of field which occur in light vibrations, but is far more complicated." and then follows with some daunting deriviations of equations describing the EM field in a dielectric body. It also states that "in dielectrics, permeability differs so little from 1 (generally only a few thousandths of 1%) that in what follows it will always be considered equal to 1." I know that the permeability of free space is 4*pi*10^-7 = 1.2566*10^-6 N/A^2 (SI units) and that free space (vacuum) is a dielectric. So, where are they getting "permeability differs so little from 1" Thanks guys.