7. Polarimetry

Question 1

What is Polarimetry?

Answer 1

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.

Question 2

What is optical rotation?

Answer 2

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.

Question 3

What is Light?

Answer 3

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.

Question 4

What is UNPOLARIZED LIGHT?

Answer 4

a light beam that contains oscillations in all directions

Question 5

What is LINEARLY (PLANE-) POLARIZED LIGHT?

Answer 5

A light beam whose electric field vector can be found in one particular plane that contains the direction of propagation.

Question 6

What are polarizers?

Answer 6

They are are optical filters that pass light of a specific polarization that corresponds to its orientation, and it blocks waves of other polarizations

Question 7

What are polarizers?

Answer 7

They are are optical filters that pass light of a specific polarization that corresponds to its orientation, and it blocks waves of other polarizations

Question 8

What is CIRCULARLY POLARIZED LIGHT?

Answer 8

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.

Question 9

How to construct circularly polarized light?

Answer 9

circularly polarized light can be composed of two perpendicular linearly polarized waves of equal amplitude and wavelength but shifted by a quarter wavelength.

Question 10

Linearly polarized light of any direction of oscillation can be produced by ___

Answer 10

rotating the polarizer.

Question 11

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)___

Answer 11

1. leading (or lagging) the horizontal component by one quarter of a wavelength,
2. right-handed or left-handed.

Question 12

Linearly polarized light can be produced by adding___ (2 things)

Answer 12

a left-handed and a right- handed circularly polarized light of identical amplitude

Question 13

The horizontal components of the left-handed and right-handed electric field vectors will cancel each other as they will have __

Answer 13

a half-wavelength phase shift (two quarter-wavelength shifts).

Question 14

However, the vertical components are in phase, thus they will___ (2 things)

Answer 14

add their components and have a greater intensity.

Question 15

he resultant vector, represented in red, oscillates in the ___ and is __

Answer 15

oscillates in the vertical plane and is linearly polarized.

Question 16

Linearly polarized light can be produced by adding___ (2 things)

Answer 16

a left-handed and a right- handed circularly polarized light of identical amplitude

Question 17

What is OPTICAL ACTIVITY?

Answer 17

ability to rotate the direction (plane) of polarization of linearly polarized light.

Question 18

In most materials (which are optically inactive, see later) the physical properties are spatially ___ and __.

Answer 18

homogeneous

anisotropic

Question 19

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 __

Answer 19

equal

Question 20

In most materials (which are optically inactive, see later) the physical properties are spatially homogeneous and anisotropic. Therefore, the speeds of ___ are equal

Answer 20

propagation of the left- and right-handed circularly polarized light

Question 21

Let us consider a left-handed circularly polarized light as it passes through two substances with different indexes of refraction

→ What is the consequence?

Answer 21

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.

Question 22

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

Answer 22

lead

Question 23

What are left- (–, levo) and right-handed (+, dextro) optically active substances?

Answer 23

They are so named because they in fact rotate the plane of polarization of light counterclockwise or clockwise, respectively.

Question 24

A chemical substance is optically active if its molecular structure is __

Answer 24

asymmetric

Question 25

What is constitution?

Answer 25

the order of atoms bound within a molecule.

Question 26

What is CHIRAL MOLECULE?

Answer 26

molecule with a spatial structure that lacks a plane of symmetry (it is asymmetric, thus it is not superposable on its mirror image).

Question 27

What is achiral molecule?

Answer 27

molecule with a spatial structure that possesses a plane of symmetry (it is symmetric, thus it is superposable on its mirror image

Question 28

What are stereoisomers?

Answer 28

molecules of the same constitution but different spatial arrangement. They can be either enantiomers or diastereomers.

Question 29

What are enantiomers?

Answer 29

a pair of stereoisomer molecules that are mirror images of each other.

Question 30

What are diastereoisomers?

Answer 30

a pair of stereoisomer molecules that are not mirror images of each other.

Question 31

What is racemic mixture?

Answer 31

a mixture of equal amounts of two enantiomers which is, therefore, optically inactive.

Question 32

What is POLARIMETER?

Answer 32

a device that measures the angle of rotation of the direction (plane) of polarization caused by optical activity.

Question 33

Is optical activity wavelength-dependent?

Answer 33

Yes

Question 34

As optical activity is wavelength- dependent (dispersion), ___ light is used.

Answer 34

monochromatic

Question 35

In this case, every chiral molecule rotates the plane of the polarization of light by \_\_

Answer 35

a small amount

Question 36

the final angle of rotation *α* is proportional to ___ (2 things)

Answer 36

the concentration *c* and the thickness *l* of the solution.

Question 37

How to calculate **Specific rotation?**

Answer 37

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).

Question 38

What is Biot law?

Answer 38

the final angle of rotation *α* is proportional to * the concentration *c* and * thickness *l* of the solution. * The specific rotation

Question 39

What is the unit for specific rotation?

Answer 39

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.

Question 40

Factors influencing the specific rotation

Answer 40

the specific rotation depends on the chemical composition of the material, the wavelength and the temperature.

Question 41

How does a polarimeter work?

Answer 41

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)

Question 42

When is the maximum intensity observed in polarimetry?

Answer 42

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

Question 43

When is the minimum intensity is observed (dark field of view) in polarimetry?

Answer 43

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.

Question 44

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.

Answer 44

(1) monochromatic (2) Condenser (3) parallel (4) Linearly (5) half-shade device (6) exact value of the rotation angle α