Polarimetry

POLARIMETRY (click on the image to see full image)

Light has two nature: (a)particle nature and (b)wave nature. Light as a wave can be shown with its ability to create interference and diffraction. The phenomenon of polarization helps to establish that waves are transverse waves and not longitudinal. Light is electromagnetic radiation. Electromagnetic radiation has two components: (a)component of electric field {E} and (b)component of magnetic field {B}. Polarization takes place in the direction of electric field.

If the electric field vector E is parallel to the reflecting surface, mostly that waves are reflected more strongly than those whose electric field vector E lies in this plan. The reflected light is partially polarized in the direction perpendicular to the plane of incidence. A particular angle of incidence at which the light with a parallel vector E to the surface is refracted as 100% polarized, is known as polarizing angle.

Polaroid filter is made up of long chain molecules oriented with their axis perpendicular to the polarizing axis. These molecules preferentially absorb light that is polarized along their length. Polaroids transmits around 80% of the intensity of a wave that is polarized parallel. Objective and reducible Retinal Nerve Fibre Layer (RFNL) measurements by quantifying changes in polarization. This is done based on different birefringence (double refraction) and it is computer based, done in real-time.

Malus’ law (named after Etienne-Louis Malus) states that, when a perfect polarizer is placed in a polarized beam of light, the intensity I of the light that passes through is given by : I = Io cos²øi (where, Io is the initial intensity, and øi is the angle between the light's initial polarization direction and the axis of the polarizer). Intensity of the transmitted light is : (a)maximum, when the transmission axes are aligned with each other, (b)lesser, when the transmission axes are at an angle of 45° with each other and (c)minimum, when the transmission axes are perpendicular to each other. The intensity of the transmitted light is the same for all orientation of the polarizing filter. For an ideal polarizing filter, the transmitted intensity is half of the incident intensity.

Optically active substances are the substances which can rotate the orientation of plane-polarized light. Anisotropic crystalline solids, samples containing an excess of one enantiomer of a chiral molecule are optically active substances. Optically active substances have different refractive indices for left- and right- circulatory polarized light, because of which optical rotation occurs. Polarimeter is used to measure the optical rotation of molecules in solution. It consists of a fixed polarizer, a polarimeter tube, an analyzer. Analyzer is a rotatable polarizer. Soviet style polarimeter looks for optical equilibrium, light in the middle comes through the sample and rotates till the contrast is equal to zero. Other type polarimeter, in this the angle that the analyzer must be rotated to return to the minimum detector signal is the optical rotation.

To find the concentration of an optically active substance, take three measurements for each sample and find their average and pay attention to numbers after comma. Using Regression analysis (graphical method), X-axis will give the concentration and Y-axis will give the (average) rotation degree.

(click on the link "POLARIMETER" to see the video)POLARIMETER

(click on the link "POLARIMETRY AND OPTICAL ACTIVITY" to see the video)POLARIMETRY AND OPTICAL ACTIVITY

(Click on the link "MALUS' LAW")MALUS' LAW