  In the present work, one analyzes two systems trying to obtain physical
conditions where some properties attributed to negative mass can be mimicked by
positive mass particles. The first one is the well-known 1/2-spin system
described by the Dirac equation in the presence of an external electromagnetic
field. Assuming some physical restrictions, one obtains that the use of
$e\rightarrow-e$ can lead to the same results as using $m\rightarrow-m$. In
particular, for a null dielectric function, it is possible to obtain a negative
mass behavior from a positive mass system composed of negatively charged
particles. The second system is based on the de Broglie matter wave. The
dispersion relation of such a wave can be negative (real or imaginary valued)
if one assumes an imaginary wavenumber. The consequence is the emergence of a
negative refractive index for positive mass particles. However, this behavior
is generally attributed to a negative mass system.
