In this paper an attempt is made to investigate how incident light propagating through a linear non-image-forming optical device, or scattered by a linear medium is depolarized using the Mueller matrix formalism. There are multiple definitions of depolarization in the literature which led to a difficulty, i.e. a non-depolarizing optical medium may decrease the degree of polarization of an incident completely polarized light. Because of this confusion, our usage is clarified. By considering incident light in the form of pure states of polarization, we are able to show that a particular Mueller matrix decomposition is advantageous for analysis of the physical mechanisms of depolarization of light propagating through linear media. We illustrate our analysis by examining several physically realizable Mueller matrices reported in the optics literature.