  Motivated by a recent finding that Fresnel's phenomenological description of
the optical activity in the chiral medium is not self-consistent, we conduct a
thorough investigation into the nature of the polarization of a plane light
wave. We demonstrate that the polarization of light is the reflection of one of
its quantum-mechanical properties, called the quasi-spin. Unexpectedly, the
quasi-spin is not an observable with respect to the laboratory coordinate
system. Instead, it is with respect to the momentum-dependent local coordinate
system. The representative operators for the quasi-spin are the Pauli matrices.
The wavefunction is the Jones vector. In order to completely determine a state
of polarization, two different kinds of degrees of freedom are needed. One is
the degrees of freedom to characterize the state of quasi-spin. They are the
Stokes parameters, the expectation values of the Pauli matrices in the state
described by the Jones vector. The other is the degrees of freedom to specify
the local coordinate system, including the propagation direction and an angle
of rotation about it. Accordingly, there are two independent mechanisms to
change the state of polarization. One is to change the state of quasi-spin in a
fixed local coordinate system. This is the traditional mechanism that can be
expressed as an SU(2) rotation of the Jones vector. The other is to change the
local coordinate system with the state of quasi-spin remaining fixed in it. At
last, we show that it is the newly-identified mechanism that accounts for the
optical activity.
