Optical Activity

Polarisation plays an important role in explaining the wave nature of electromagnetic waves. While studying the polarisation concept we encounter many interesting concepts regarding the wave nature of the electromagnetic waves, one among them is optical activity. Optical activity is a phenomenon that describes the ability of rotation, thus optical activity is also known as optical rotation. Optical activity is different from polarisation. The optical activity corresponds to the property of some materials to rotate the plane of polarization of light waves.

Optical rotation

• Let us have a look at what optical rotation is. So, optical activity or optical rotation is the ability of a compound to rotate the plane of polarized light, and the compounds having this ability to rotate the plane of polarized light are known as optically active materials.

• The optical rotation of substances is due to the interaction of the electromagnetic radiation of polarized light with the unsymmetric electric fields generated by the electrons in a chiral molecule. Optical activity is usually found in organic substances. For example, the sugar solution is optically active, it exhibits optical rotation on observing through the polarimeter. Other examples of optically active substances are turpentine, sodium chlorate, cinnabar, etc...

• Any substance or compound is said to be optically active when the linearly polarized light is being rotated when it is passing through it. 

• The optical rotation or optical activity is the angle through which the plane of polarization is rotated when polarized light passes through a layer of a liquid (such as sugar solution or in other words diluted sugar solution).

• Optical rotation is the effect that is determined by the concentration of chiral molecules and their molecular structure in a substance. Every optically active substance will have its specific rotation. Depending on the concentration level the optical activity will be either increasing or decreasing.

What is Optical Activity - Define Optical Activity

In 1811, French physicist Francious Arega observed that when a plane polarised light passed through some materials in particular through some crystals such as quartz, the plane of emerging light is not the same as the plane of the incident light. These crystals rotated the plane of polarization of incident light. This phenomenon of rotation is known as an optical activity or optical rotation. 

Optically Active Meaning

The substances that exhibit such rotations are known as optically active substances. To understand what is optical activity or optical rotation we will explain a unique experiment. The optical rotation is measured through a polarimeter. 

The optical activity of optically active substances is studied by the polarimeter. Polarimetry Gives the measurement of rotation of plane-polarized light by an optically active substance. The instrumentation of the polarimeter is given below -

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• So let us understand how optical activity works. Now, Consider an ordinary source of light that is placed in front of a single to provide rectilinear unpolarized light. The unpolarized light is further passed through a polarizer (Which is a combination of Nicol prism). 

• As soon as the light enters the Nicol prism they undergo a double refraction process i.e, incident light gets split into two ordinary rays and the extraordinary ray. Both E-ray and O-ray are plane polarised and perpendicular to each other. After passing through the polarizer we get plane-polarized light. 

• The vibrations of the ordinary ray will be perpendicular to the plane of paper through which we are observing and the vibration of the extraordinary ray will be in the direction of the plain paper. 

• So, the ordinary gets eliminated after getting total internal reflection inside the Nicol prism and an extraordinary ray will be passed through the Nicol prism such that the vibration of its electric field will be in a single direction.

• On passing the plane-polarized light through an analyzer, we get to see no emerging light. For the same experimental set up if we place an optically active substance between polarizer and analyzer we get to see an emerging light. It explains the fact that the plane-polarized light is rotated. 

Generally, the ordinary light is unpolarized which means each light wave oscillates randomly. In the polarimeter experiment, initially, the unpolarized light is converted into polarized light by using polarizing filters or Nicol prism polarizer. Later this polarized light is passed through the polarimeter tube in which the sample (optically active) is kept. This polarized light gets rotated and gives the result on a Nicol prism analyzer.

Optically Active Substances are Classified in Two Types :

1. Dextrorotatory Substances: The dextrorotatory substances are also known as the right substances. Dextrorotatory Substances are those optically active substances that rotate the plane of polarization of the light towards the right and are known as right-handed or dextrorotatory. In other words, if a substance rotates the plane-polarized light to the right or clockwise direction, such substances are known as the Dextrorotatory substances.

2. Laevorotatory Substances: Laevorotatory Substances are the type of substances that rotate the plane of polarization of the light toward the left and are known as left-handed or Levorotatory. The Levorotatory substances are those optically active substances that rotate the plane of polarization of the light toward the left are known as left-handed. In other words, if a substance rotates the plane-polarized light to the left or counterclockwise direction, such substances are known as the Dextrorotatory substances.

The Optical Rotation Definition

The phenomenon of optical rotation was studied in detail by Biot in the year 1815, and he proposed the laws corresponding to optical rotation. The laws of optical rotations are given as follows:

1. The amount of optical rotation produced by optically active crystals or substances is directly proportional to the thickness of the crystal or the path length traversed in its rotation.

θ ∝ l

Where,

θ - The angle of optical rotation

l - The path length

2. The amount of optical rotation is directly proportional to the concentration of the optically active solution.

θ ∝ C

Where,

C - Concentration of the optically active solution

3. The angle of optical rotation is inversely proportional to the square of the wavelength of the light used.

θ \[ \alpha \frac{1}{\lambda^{2}}\]

       Where,

\[\lambda\] - The wavelength of the light used

Combining all these points we find that the angle of rotation or formula for rotation is given by:

θ ∝ Cl

Where,

θ - The angle of optical rotation

l - The path length

C - Concentration of the optically active solution

Further, the proportionality constant is replaced by a constant S known as the specific rotation. Then the formula for optical rotation or angle of optical rotation is given by:

θ = S Cl

Where,

S-Specific rotation

Note: The angle of rotation is also depending upon the temperature and nature of the optically active substances.

Specific Optical Rotation

Specific rotation is a characteristic property of an optically active substance and it is the standard measurement for optical rotation for that optically active substance. Specific rotation gives the change in the orientation of plane-polarized light per unit distance of the cell and per unit concentration of the sample when light is passed through that sample. Specific rotation is an intrinsic property of the substance.

So specific rotation of an optically active substance is given by:

\[S = \frac{\theta}{Cl}\]

Where,

\[\theta\] -The angle of optical rotation

l - The path length

C - Concentration of the optically active solution

S - Specific rotation

The basic difference between optical rotation and specific optical rotation is that the optical rotation of the substance is the rotation of plane-polarized light by a substance. The optical rotation can be either clockwise or anticlockwise. The compounds that are capable of exhibiting this rotation are enantiomers. The standard measurement for the amount of optical rotation of a specific substance is called specific rotation.

Applications of Optical Rotation or Optical activity

• Optical rotation is used to determine the percentage of the optically active substance in the solution.

• The sugar level in the urine of a diabetic person is determined by calculating the angle of rotation of the plane of polarization.

• Optical activity is a function of time and it is used to determine kinetic reactions.

• The optical rotation is also utilized to plot optical rotatory dispersion curves for various ranges of wavelengths; this helps in analysing molecular structure.

• The optical activity is measured on a layer of suitable thickness at the wavelength specified in the monograph.

Did you Know

The specific rotation of every material will be different from one another. Every organic substance will have different specific rotations. The optical activity chemistry is of major interest. 

Conclusion

Many students will have doubts regarding what is optical activity in chemistry, well optical activity is found mostly in chiral substances hence it is of equal importance in chemistry as well.