POLARIMETRY - Discussion

Discussion:
Source of monochromatic light is placed at the focus of convex lens. The beam, rendered parallel by lens, falls on polarizer. After passing through polarizer the light becomes plane polarized. The polarized light beam passes through a half-shade device and then through a tube containing optically active solution. The transmitted light passes through another Nicol which can be rotated about the direction of propagation of light as axis and its rotation can be read on a circular scale graduated in degree, with the help of a Vernier scale. 
This experiment is used for the measurement of the angle of rotation of optically active substance in solution. That is, angle through which the plane of the polarized light is rotated on passing through a specific length of solution of known concentration. 
In the experiment, sugar is dissolved completely in water so that concentration of solution is exact. Air bubbles are removed to reduce errors. Temperature during experiment is recorded because optical activity depends on temperature. Failure to judge the equality of brightness will introduce errors. To minimize those errors, four readings should be taken for each concentration at intervals of ≈180 degrees. 
Angle of rotation of water is zero. Now, the angle of rotation of sugar solution is the angle at which intensities of both (right and left) sides are equal and bright. This is by half shade principle.

POLARIMETRY - Q13

13) Can you perform this experiment using white light instead of sodium light?

Ans: Yes, we can perform the experiment using white light instead of sodium light.

POLARIMETRY - Q12

12) Distinguish between natural and magnetic rotation of the plane of polarization.

Ans: When a paramagnetic gas is placed in a magnetic field it becomes "circularly birefringent," that is, the vapor has different indices of refraction for left- and right-circularly polarized light. For light which is initially linearly polarized, the different indices of refraction lead to a rotation of the plane of polarization of light which is dependent on the magnetic field strength and the pathlength. Where natural plane of polarization is the plane perpendicular to magnetic field.

POLARIMETRY - Q11

11) How are the specific rotations of pure liquids and pure solids defined?

Ans: 

For pure liquids:

In this equation, l is the path length in decimeters, and ρ is the density of the liquid in g/mL, for a sample at a temperature T (in degrees Celsius) and wavelength λ (in nanometers).

For pure solids:

The specific rotation of a solid is defined as the optical rotation in degrees produced by a 1 mm thickness of the solid. For a compound in solution the specific rotation [α] is given by 

where α is the measured rotation in degrees, d the path length of the solution in mm and c the concentration in g/100 cm3.

POLARIMETRY - Q10

10) What do you mean by (i) molecular rotation, and (ii) rotatory dispersion?

Ans: 

• Molecular rotation: The value equaling 1/100 of the product of the specific rotation of an optically active compound and its molecular weight.
• Rotatory dispersion: The variation in rotation with the wavelength of the light is called rotatory dispersion.

POLARIMETRY - Q9

9) Define specific rotation. Does it depend on the wavelength of light used and temperature of the solution?
Ans: The specific rotation of a chemical compound [α] is defined as the observed angle of optical rotation α when plane-polarized light is passed through a sample with a path length of 1 decimeter and a sample concentration of 1 gram per 1 millilitre.

where l is the path length in decimeters, and ρ is the density of the liquid in g/mL, for a sample at a temperature T (in degrees Celsius) and wavelength λ (in nanometers).

Yes, specific rotation depends on the wavelength of light used and temperature of the solution.

• As the wavelength increases, optical activity decreases.
• As the temperature increases, optical activity decreases. Temperature dependence of specific rotation is for sugar solutions as follows:

a_(t) = a_{(20 C)}[1 – 0.000471 (t – 20.0)]

POLARIMETRY - Q8

8) How does the optical activity depend on the state of the substance?
Ans: Optical activity is related to the structure of substance. Right handed and left handed crystals show different rotations. Some substances show optical activity only in crystal state, some only in solution state, some in both states. This is how optical activity depends on the state of the substance.

POLARIMETRY - Q7

7) What type of crystals does exhibit optical activity?

Ans: Polarisation occurs in solutions of chiral molecules such as sucrose (sugar), solids with rotated crystal planes such as quartz, and spin-polarized gases of atoms or molecules. Thus anisotrophic chiral crystals exhibit optical activity.

POLARIMETRY - Q6

6) How does the optical activity depend on (i) wavelength, and (ii)temperature?
Ans: 
(i) As the wavelength increases, optical activity decreases.
(ii) As the temperature increases, optical activity decreases. Temperature dependence of specific rotation is for sugar solutions as follows: 

a_(t) = a_{(20 C)}[1 – 0.000471 (t – 20.0)]

POLARIMETRY - Q5

5) What are the factors on which the optical activity of a substance depends?
Ans: The optical activity of a substance depends on temperature, wavelength of light used and concentration of substance. Of course it also depends on nature of substance.

POLARIMETRY - Q4

4) What is an “optically active substance”?
Ans: Anisotropic crystalline solids, and samples containing an excess of one enantiomer of a chiral molecule, can rotate the orientation of plane-polarized light. Such substances are called optical active substances.
Eg. sucrose, glucose, some amino acids & sugars

POLARIMETRY - Q3

3) What do you understand by polarized light and the plane of polarization?
Ans: The figure shows a light wave propagating along the X-axis. The electric field vector E is confined to the YZ-plane. Let us assume that it is along the Y-axis. The magnetic field vector B is along the Z-axis. Then the wave light is polarized light and XY-plane is the plane of polarization.

POLARIMETRY - Q2

2) What is optical axis? Name uniaxial and biaxial crystals?
Ans: An optical axis is a line along which there is some degree of rotational symmetry in an optical system such as a camera lens or microscope. The optical axis is an imaginary line that defines the path along which light propagates through the system.

• Crystal belonging to the Tetragonal, Hexagonal and Trigonal system has only one optic axis, parallel to the crystallographic "C" axis direction, and is known as uniaxial gemstones
• Crystals belonging to the Orthorhombic, Monoclinic and Triclinic systems, have two directions of single refraction i.e. two optic axis and hence these crystals are termed biaxial gemstones

POLARIMETRY - Q1

1) How does polarized light differ from the ordinary light?
Ans: Polarization is the direction of oscillation of the electric field in an EM wave. Ordinary (unpolarized) light has random polarization, meaning it will have statistically equal components of
polarization in any direction. Polarized light has coherent polarization, meaning the polarization is in one constant direction for the light (polarization can be linear, circular, or elliptical). Light can be partially polarized as well.