PowerPoint Presentation

1. Understanding Circular DichroismThe basics of polarisation

To really understand circular dichroism, one must first understand the basics of polarisation.Linearly polarised light is light whose oscillations are confined to a single plane. All polarised light states can be described as a sum of two linearly polarised states at right angles to each other, usually referenced to the viewer as vertically and horizontally polarised light. This is shown in the animations below.1. Understanding Circular DichroismThe basics of polarisationTo really understand circular dichroism, one must first understand the basics of polarisation.Linearly polarised light is light whose oscillations are confined to a single plane. All polarised light states can be described as a sum of two linearly polarised states at right angles to each other, usually referenced to the viewer as vertically and horizontally polarised light. This is shown in the animations below. Vertically polarised light

Vertically Polarised Light

Vertically Polarised LightHorizontally polarised light

If for instance we take horizontally and vertically polarised light waves of equal amplitude that are in phase with each other, the resultant light wave (blue) is linearly polarised at 45 degrees, as shown in the animation below: Polarised light at 45 degrees45 Degree Polarised Light

If the two polarisation states are out of phase, the resultant wave ceases to be linearly polarised. For example, if one of the polarised states is out of phase with the other by a quarter-wave, the resultant will be a helix and is known as circularly polarised 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 polarised light and circularly polarised light is termed a quarter-wave plate. A quarter-wave plate is birefringent, i.e. the refractive indices seen by horizontally and vertically polarised light are different. A suitably oriented plate will convert linearly polarised light into circularly polarised 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-hand circularly polarized lightLeft Circularly Polarised (LCP) Light

Right-hand circularly polarized light

Right Circularly Polarised (RCP) Light

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