Polarization Microscopy - Biomedical Optical Phase Microscopy and Nanoscopy - page 323

15.7.7 Fast Axis
The fast axis describes an orientation in a birefringent material. For a given propagation
direction, light that is polarized parallel to the fast axis experiences the lowest refractive
index, and hence travels the fastest in the material (for the given propagation direction).
See also Section 15.7.16 .
15.7.8 Optic Axis
The optic axis refers to a direction in a birefringent material. Light propagating along the
optic axis does not change its polarization, hence for light propagating along the optic axis,
the birefringent material behaves as if it were optically isotropic.
15.7.9 Polarized Light
A beam of light is said to be polarized when its electric field is distributed nonrandomly in
the plane perpendicular to the beam axis. In unpolarized light, the orientation of the electric
field is random and unpredictable. In partially polarized light, some fraction of the light is
polarized, while the remaining fraction is unpolarized. Most natural light is unpolarized
(sun, incandescent light) but can become partially or fully polarized by scattering,
reflection, or interaction with optically anisotropic materials. These phenomena are used to
build devices to produce polarized light (see Section 15.7.13 ).
15.7.10 Linearly Polarized Light
In a linearly polarized light beam, the electric field remains oriented along a single
direction that is perpendicular to the propagation direction.
15.7.11 Circularly Polarized Light
In circularly polarized light, the electric field direction rotates either clockwise (right
circularly) or counterclockwise (left circularly) when looking toward the source. While the
field direction rotates, the field strength remains constant. Hence, the endpoint of the field
vector describes a circle.
15.7.12 Elliptically Polarized Light
In elliptically polarized light, as in circularly polarized light, the electric field direction
rotates either clockwise or counterclockwise when looking toward the source. However,
while the field direction rotates, the field strength varies in such a way that the end point of