Page 98 - Tandem Techniques
P. 98
Page 79
circularly polarized light is then passed through the cell and the intensity of the transmitted light
monitored by the sensor.
Figure 2.21
A Modern CD Spectrometer
Polarization Modulation.
The alternate production of left or right circularly polarized light, which is called polarization
modulation, is an essential process for CD measurement. Now, a linearly polarized light beam can be
said to be the resultant of two orthogonal, in phase, linear, light beams. Consider a block of isotropic
fused silica that is rendered birefringent by pressure exerted along the (x) or (y) axis as shown in Figure
2.22.
Under these condition the refractive indexes (nx and ny) will differ, and
nx „ ny
If the light beam passing through the block is oriented with its resultant axis at 45° to the pressure axis,
one of the components will travel faster through the medium than the other. If nx > ny, then the (x)
component of
the light beam will travel more slowly. If the retardation is exactly
the emergent light beam will be right circularly polarized.
If the retardation is then the emergent light beam will be left circularly
polarized.
