I have a basic question on Mueller matrix which I came across upon reading through the Handbook of Optic, chapter 22 polarimetry. It says that :-
when a polarization element with Mueller matrix M is rotated about the beam of light by an angle theta such that the angle of incident is unchanged, the resulting Mueller matrix M is :
= R(theta) M R(-theta)
= [matrix elements of R(theta)] [matrix elements of M] X [matrix elements of R(-theta)]
My question is, what does the "X" mean here..
Sorry if this is too basic, I've just started brushing up my matrix since college.
Appreciate any help.