Plane polarized light PDF

Preview

Summary

Lecture notes on plane polarised light topic....

Description

1 Plane'polarized'light' Light waves have two components (and electric field component and a magnetic field component) that propagate at the same speed, in the same direction, in phase but at right angles to each other. Normal (unpolarized) light is comprised of waves with the electric field pointing in all possible directions. It is possible to filter this unpolarized light so that we allow only waves with a single orientation of the electric field vector (and, therefore only a single orientation of the magnetic field vector) to pass through the filter. This gives us plane polarized light. The “polarizers” are usually Nicol prisms. These are rhombohedral crystals of calcite (CaCO3) that are essentially cut and then glued back together

(it’s

sophisticated

a

little

than

that).

more The

refraction that takes place at the join between the two parts of the prism is what

gives

rise

to

the

plane

polarization.

1.1.1 Rotating the plane of plane polarized light Certain chemical species, including as amino acids, are “optically active”. This means that when plane polarized light passes through a sample containing such molecules, the plane of polarization is rotated:

Ø Helical structure ^

This optical activity comes from the presence of chiral centres in the molecule. The general criterion for optical activity is that the molecule and its mirror image must not be superimposable.

The angle recorded is the angle of rotation, a , of the analyser that is required in order for the plane polarized light to be visible at the eye-piece. This angle is then used to calculate the specific rotation T

[a ]l

=

a lc

units of specific roatation is degrees per decimetre cm^3 per gram (deg.dm^-

1.cm^3.g^-1) where c is the concentration of the optically active species in (rather unusually) grams per 100 cm3, l is the path length of the sample cell, T is the temperature (in °C) and l is the wavelength of the monochromatic light passing through the sample. If the medium causes the plane of polarisation to rotate to the right, it is dextrorotatory (+) and if the rotation is to the left it is levorotatory (-).

The plane of polarisation rotates because in an optically active medium, the refractive index

n ( l ) is different for left and right circularly polarised light (wavelength specific), with the angle of rotation being determined by the difference a =

p ( nL - nR ) l in radians l

A spectropolarimeter measures how the optical rotation varies with wavelength giving an optical rotatory dispersion curve (ORD), which is often quantified by the molar rotation T

T

[F ]l = M [a ]l /100 , where M is the molar mass in g mol-1.

For proteins and nucleic acids,

measurements may be expressed in terms of per mole of repeating unit, or even per mean residue weight. Ø Use to determine protein concentration

From Chang rec. text

The optical rotatory dispersion has the interesting property that it changes sign as the wavelength is scanned through an absorption band. So, consider a molecule with an absorption centred on some wavelength l a . If the sample rotates plane polarized light through a positive

angle (clockwise from the perspective of the observer) for l < l a then it will rotate the plane of polarization through a negative angle for l > l a . This is known as the Cotton effect.

The main Cotton effect for proteins and polypeptides occurs for wavelengths around 200 nm due to absorption by the peptide bond. For nucleic acids it results from excitations of the nucleotide bases at around 250 nm to 275 nm....