Chemistry Reference
In-Depth Information
Figure 4.7. Vibration directions of the ordinary and the extraordinary rays in a positive
uniaxial liquid crystal. OX is optic axis.
X
(
R
)
.
R
.
S
R
N
RN
X
X
X
O
.
.
O
.
.
O
O
P
,
P
,
.
P
,
,
P
P
P
(
P
)
P
(a)
(b)
(c)
(d)
Figure 4.8.
Linear-polarized light passing through a positive uniaxial liquid crystal.
by dark spots on OS. OR is an extraordinary ray. The vibration direction of
OR is in the plane ROX and parallel to the wave front RN. This is denoted
with strokes across OR, which are perpendicular to the wave normal ON.
In studies of liquid crystals with POM, a polarizer is used (Figure 4.4)
so that the source light is linear-polarized with vibration direction deter-
mined by the polarizer. If PP
is the vibration direction of the entering
light, the result will depend on the relative orientation of PP
with the axis
and principal section of the liquid crystal. In Figure 4.8, a few cases for
a positive uniaxial liquid crystal are shown. When the incident is paral-
lel to the optic axis (Figure 4.8(a)), the refracted light will vibrate in the
same direction as PP
. The light emerging from the liquid crystal will also
vibrate in this direction. Thus, if a second polarizer (the analyzer) is used
on the detecting side as shown in Figure 4.4 and if the vibration direc-
tion of the analyzer (AA
, not showing in Figure 4.8) is at a right angle to
