The basics of polarisation
To really understand what circular dichroism is, you must first understand the basics of polarization
Linearly polarized light is light whose oscillations are confined to a single plane. All polarized light states can be described as a sum of two linearly polarized states at right angles to each other, usually referenced to the viewer as vertically and horizontally polarized light. This is shown in the animations below:
If the two polarization states are out of phase, the resultant wave ceases to be linearly polarized. For example, if one of the polarized states is out of phase with the other by a quarter-wave, the resultant will be a helix and is known as circularly polarized light (CPL). The helices can be either right-handed (R-CPL) or left-handed (L-CPL) and are non-superimposable mirror images.
The optical element that converts between linearly polarized light and circularly polarized light is termed a quarter-wave plate. A quarter-wave plate is birefringent, i.e. the refractive indices seen by horizontally and vertically polarized light are different. A suitably oriented plate will convert linearly polarized light into circularly polarized light by slowing one of the linear components of the beam with respect to the other so that they are one quarter-wave out of phase. This will produce a beam of either left- or right-CPL.
