7. Polarimetry Flashcards
What is Polarimetry?
an analytical method which measures the angle of rotation of the direction (plane) of vibration of linearly polarized light after it passed through a given length of optically active material.
What is optical rotation?
the property of materials with spatial asymmetry (chiral molecules). Polarization plane is rotated either to the left (counter-clockwise) by levorotatory substances or right (clockwise) by dextrorotatory substances.
What is Light?
an electromagnetic wave characterized by the electric and magnetic field vectors (E, H) which oscillate in phase and in perpendicular to each other and to the direction of wave propagation.
What is UNPOLARIZED LIGHT?
a light beam that contains oscillations in all directions
What is LINEARLY (PLANE-) POLARIZED LIGHT?
A light beam whose electric field vector can be found in one particular plane that contains the direction of propagation.
What are polarizers?
They are are optical filters that pass light of a specific polarization that corresponds to its orientation, and it blocks waves of other polarizations
What are polarizers?
They are are optical filters that pass light of a specific polarization that corresponds to its orientation, and it blocks waves of other polarizations
What is CIRCULARLY POLARIZED LIGHT?
a light wave whose electric field vector has constant amplitude, and its tip follows a helical path around the direction of propagation.
-> Construction: circularly polarized light can be composed of two perpendicular linearly polarized waves of equal amplitude and wavelength but shifted by a quarter wavelength.
How to construct circularly polarized light?
circularly polarized light can be composed of two perpendicular linearly polarized waves of equal amplitude and wavelength but shifted by a quarter wavelength.
Linearly polarized light of any direction of oscillation can be produced by ___
rotating the polarizer.
If the vertical component is (1)___ the resultant electric field vector has a constant amplitude and rotates as if the radius of a circle, hence this is circularly polarized light. The electric field vector of constant size rotates around the propagation axis. Thus, the tip of the electric field vector follows a helical path, which is either(2)___
- leading (or lagging) the horizontal component by one quarter of a wavelength,
- right-handed or left-handed.
Linearly polarized light can be produced by adding___ (2 things)
a left-handed and a right- handed circularly polarized light of identical amplitude
The horizontal components of the left-handed and right-handed electric field vectors will cancel each other as they will have __
a half-wavelength phase shift (two quarter-wavelength shifts).
However, the vertical components are in phase, thus they will___ (2 things)
add their components and have a greater intensity.
he resultant vector, represented in red, oscillates in the ___ and is __
oscillates in the vertical plane and is linearly polarized.
Linearly polarized light can be produced by adding___ (2 things)
a left-handed and a right- handed circularly polarized light of identical amplitude
What is OPTICAL ACTIVITY?
ability to rotate the direction (plane) of polarization of linearly polarized light.
In most materials (which are optically inactive, see later) the physical properties are spatially ___ and __.
homogeneous
anisotropic
In most materials (which are optically inactive, see later) the physical properties are spatially homogeneous and anisotropic. Therefore, the speeds of propagation of the left- and right-handed circularly polarized light are __
equal
In most materials (which are optically inactive, see later) the physical properties are spatially homogeneous and anisotropic. Therefore, the speeds of ___ are equal
propagation of the left- and right-handed circularly polarized light
Let us consider a left-handed circularly polarized light as it passes through two substances with different indexes of refraction
→ What is the consequence?
the wavelength shortens more in the substance of the larger index of refraction
→ the phase of the resultant vector leads compared to the substance with the smaller index of refraction.
In an optically active material, the left- and right-handed circularly polarized components of the same amplitude and wavelength experience different refractive indices (e.g.,: nleft > nright).
→ The phase of the left-handed circularly polarized light will __ the right-handed one
lead
What are left- (–, levo) and right-handed (+, dextro) optically active substances?
They are so named because they in fact rotate the plane of polarization of light counterclockwise or clockwise, respectively.
A chemical substance is optically active if its molecular structure is __
asymmetric
What is constitution?
the order of atoms bound within a molecule.
What is CHIRAL MOLECULE?
molecule with a spatial structure that lacks a plane of symmetry (it is asymmetric, thus it is not superposable on its mirror image).
What is achiral molecule?
molecule with a spatial structure that possesses a plane of symmetry (it is symmetric, thus it is superposable on its mirror image
What are stereoisomers?
molecules of the same constitution but different spatial arrangement. They can be either enantiomers or diastereomers.
What are enantiomers?
a pair of stereoisomer molecules that are mirror images of each other.
What are diastereoisomers?
a pair of stereoisomer molecules that are not mirror images of each other.
What is racemic mixture?
a mixture of equal amounts of two enantiomers which is, therefore, optically inactive.
What is POLARIMETER?
a device that measures the angle of rotation of the direction (plane) of polarization caused by optical activity.
Is optical activity wavelength-dependent?
Yes
As optical activity is wavelength- dependent (dispersion), ___ light is used.
monochromatic
In this case, every chiral molecule rotates the plane of the polarization of light by __
a small amount
the final angle of rotation α is proportional to ___ (2 things)
the concentration c and the thickness l of the solution.
How to calculate Specific rotation?
calculated as the constant of proportionality, where..
- the lower index refers to the wavelength (usually the D line of sodium at λ = 589 nm)
- the upper index is the temperature (T = 20°C).
What is Biot law?
the final angle of rotation α is proportional to
- the concentration c and
- thickness l of the solution.
- The specific rotation
What is the unit for specific rotation?
the angle of rotation in degrees of the plane of polarization of a ray of monochromatic light that passes through a tube 1 decimeter long containing the substance in solution at a concentration of 1 gram per millimeter in a polarimeter.
Factors influencing the specific rotation
the specific rotation depends on the chemical composition of the material, the wavelength and the temperature.
How does a polarimeter work?
Unpolarized light from the light source (Fig. 8) is first linearly polarized in the vertical direction by the 1st fixed polarizer.
→ This vertical, linearly polarized light passes through the polarimeter tube
→ The plane of the polarized light waves is rotated by angle α compared to the initial direction.
→ The angle of rotation is measured by turning the 2nd turnable polarizer (analyzer)
When is the maximum intensity observed in polarimetry?
If the orientation of the analyzer coincides with the polarization direction of the transmitted light, maximum intensity is observed (bright field of view).
→ The angle of rotation can be read in degrees from the scale as the angle of the rotation of the analyzer
When is the minimum intensity is observed (dark field of view) in polarimetry?
If the analyzer is rotated into the crossed position, i.e., at 90 degrees compared to the previous case, minimum intensity is observed (dark field of view). The angle of rotation α can be read on the properly set scale.
THE POLARIMETER SETUP
(1) _______ light source is provided by a sodium lamp (of yellow color) positioned in the lamp housing with a (2)_____ lens that forms (3)_____ rays.
(4) ______ polarized incident light is produced by the fixed 1st polarizer. Optically active sample solutions are poured into a removable glass tube of given length, which has screw caps with glass windows at both ends.
After passing through the solution, light reaches the turnable analyzer where (5)______ helps finding the (6)______(Fig. 10) by using a telescope.
(1) monochromatic
(2) Condenser
(3) parallel
(4) Linearly
(5) half-shade device
(6) exact value of the rotation angle α
