Understanding Linear and Circular Polarized Light

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:

Vertically Polarized Light

Horizontally Polarized Light

45 Degree Polarized Light

If for instance we take horizontally and vertically polarized light waves of equal amplitude that are in phase with each other, the resultant light wave (blue) is linearly polarized at 45 degrees, as shown in the animation to the left.

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.

Left Circularly Polarized (L-CPL) Light

Right Circularly Polarized (R-CPL) Light​

The difference in absorbance of left-hand and right-hand circularly polarized light is the basis of circular dichroism.  A molecule that absorbs L-CPL and R-CPL differently is optically active, or chiral.