Polarimetry (2. LF UK)

Definition
The polarimeter examines the polarization of transverse waves, mostly of lightwaves.

Principle
Light waves are electromagnetic waves which propagate in the room. The direction of oscillation of every single wave is differently and randomly distributed in all directions.

Lightwaves, which only oscillate in one direction are called polarized lightwaves.

Polarization of light can happen by reflexion, refraction and through a polarization filter.

For analysis of optical active substances (=substances which rotate/change the direction of polarized light), so called Polarimeters are used. These apparatuses consist of a sodium vapor lamp which beams out monochromatic light. This light is sent through a polarizer, which serves like a grid, only letting light through which oscillates in the direction of the grid.

If the analyzed substance has got the same "grid-direction" like the polarizer, the polarized light will pass through completely in the same direction like the light beam (substance is not optically active); if the analyzed substance is optically active, the direction of polarized light will be turned by a specific rotation angle specific for the substance, so that you will see a light beam which has got another direction than the "starting-beam".

Branches of medicine
Polarimetry is in indirect contact with almost all medical specialties, this method is in basic charge of medical examinations. It is commonly known by tests such as Blood Urea Nitrogen test (gives info about kidneys and liver functions), Creatinine Blood test (measures level of Creatinine), Transaminases test (Liver function), Hemogram (checks for blood disorders), Glycaemia (concentration of glucose). Polarimetry is also able to determine the concentration of fructose, sucrose and glucose and is used to diagnose diabetes (excessive concentration of glucose in the human body)

Practical medicine (diagnosis+therapy)
When vision loss-diabetes symptom-occurs, the patient will attend an ophthalmologist. Afterwards, the doctor may conclude that this patient has diabetic retinopathy (damage to the retina due to diabetes) through observation of the retina and in order to complete the diagnosis will ask for a Glycaemia test (see above), which will confirm the diagnosis.

Also Urinalysis is based on the same principle as polarimetry. If one of possible diseases observed in patient is Renal Insufficiency (poor functioning of kidneys), the doctor arranges a series of diagnosis tests, among them is the Urinalysis, where he will discover (using polarimetry method) the concentration of protein in the urine.

Polarimetry is used not only for diagnostic but also as a therapy control (don’t confuse with therapy), where it is possible to measure the concentrations and figure out if the therapy is being effective or not.

Theoretical Introduction
Light is a transverse wave (A transverse wave is a moving wave that consists of oscillations occurring perpendicular (or right angled) to the direction of energy transfer). The direction of propagation is perpendicular to the vectors of both the electric and magnetic field.

There are many theories to explain the propagation of light, however the most important ones are:

 

1) Planck's Radiation study of radiant Cavity (approx. 1900):

He assumed that the atoms that make up these cavity walls behave like tiny electromagnetic oscillators.

2)R. A. Millikan ( 1868 - 1953)

Nobel prize winner in 1923 for his study of the photoelectric effect.

a) Photoelectric effect is largely a surface phenomenon. (The photoelectric effect refers to the emission, or ejection, of electrons from the surface of, generally, a metal in response to incident light.)

b) Einstein's photon hypothesis meets the objections raised against the wave-theory interpretation of the photoelectric effect.

 

3)Seventeen centuries, Huygen's Principle and the law of refraction:

Using refraction and reflection, Huygen presented the light as a wave front. The application of Huygen's principle to refraction requires that if a light ray is bent toward the normal in passing from air to an optically dense medium then the speed of light in that optically dense medium 'glass' must be less than that in air. The theory is the same for the reflection of a plane wave.

 

4) Proposed theory in the past :

In the past some represented the light travelling as if the source was a machine-gun sending projectiles. The theory felt short of explaining the phenomenon of refraction and was dropped.

Polarization is based on the wave nature of light, it is a phenomenon of transforming unpolarized light into polarized light. When the direction of vibration of electric field vector takes place in all possible directions, then the light is said to be unpolarized, and after undergoing polarization, the light waves vibrate in one single plane.

These are a few ways to polarize light:-

1.       Polarization by transmission

2.       Polarization by reflection

3.       Polarization by refraction

4.       Polarization by scattering

 

Polarization can be done using polarizers. A polarizer is a machine that lets light of a certain polarization pass and blocks the rest of the light waves of other polarizations. Specific rotation is a fundamental property of chiral substances that is expressed as the angle to which the material causes polarized light to rotate at a particular temperature, wavelength and concentration.

Specific rotation is denoted by [α].

T- stands for the temperature

λ is the wavelength of light employed

c-concentration in grams per milliliter (for pure substances the density) or grams per 100 milliliters. The solvent (often ethanol, methanol, DSMO, acetone, water, etc.)

Specific rotation may also be expressed as degrees per mole of the substance where the conditions of measurement (i.e. solvent, light source, and path length) are also specified.

Optical rotation or optical activity is the rotation of the plane of polarization of linearly polarized light as it travels through certain materials. Optical rotation can only be observed in chiral objects or optically active substance. Optically active substances are those which can rotate plane-polarized light. Chirality is a property of a substance according to which it’s mirror image can’t be superimposed on it. Crystals are the most abundant naturally anisotropic occurring optically active substance. For example, the nicol prism is made up of two prisms of calcite cemented with Canada balsam. An "axis of anisotropy" is defined as the axis along which isotropy is broken. These substances also show the property of birefringence.(Birefringence is the optical property of a material having a refractive index that depends on the polarization and propagation direction of light.)

Evaluation of specific rotation of a substance (D-Glucose) solution with a given concentration
1) Prepare the measuring equipment and an enclosed 20 cm long cuvette with D-Glucose solution with concentration 10g/100ml of the solution.

2) Insert the cuvette to the polarimeter

3) Measure the optical rotation of the solution

4) Repeat step 3 five times

5) Write the results to the report table

6) Calculate average value and standard deviation

7) Calculate specific optical rotation of D-glucose  *Recommended relation 100α = [α ] · l · c   c in g/100ml   l = cuvette length -Specific optical rotation- the change in orientation of monochromatic plane-polarized light, per unit distance–concentration of the product, as the light passes through a sample of a compound in solution.

Evaluation of concentration of an optically active substance
1) Enclosed 10 cm long cuvette with D-Glucose has an unknown concentration.

2) Insert the cuvette to the polarimeter 3) Measure the optical rotation of the solution

4) Repeat step 3 five times

5) Write the results to the report table

6) Calculate the average value of optical rotation [α ] and standard deviation σ

7) Use your results and specific rotation value (from 1st part)

8) Find the concentration

Future of Polarimetry
The future plans of polarimetry are to make polarimeters smaller, cheaper, mobile, simpler and more precise. Every year new advanced models get developed as further researches are conducted. Scientists are claiming that in the near future a microchip sized polarimeter will be invented. The possible invention of a quantum computer will allow quantum components to be used in polarimetry, which will be a major development. The field of astro polarimetry is progressing quickly due to the need of an improved technology to make new discoveries which are more precise and informative. However, progress with polarimetry is very time consuming and difficult. Hence, big technological leaps are not expected to happen, rather slow and steady progress over time.