Q. What is "optical activity and optical rotation"? Write the laws of optical activity of optically active solution.

Optical Activity

If a plane polarized light is made to pass through certain substances, it is found that the plane of polarization of the emergent light is not the same as that of the incident light and it is rotated through a certain angle.

This property of rotating the plane of polarization is called optical activity or rotator polarization or more commonly as rotation of plane of polarization. The substances showing such property are known as optically active substances.

There are two types of optically active substances:

(i)    Dextrorotatory or right handed: Those substances which produce clockwise rotation of the plane of polarization (when the observer is looking towards light traveling toward light traveling towards him, the plane of vibration is rotated in clockwise direction), are known as Dextrorotatory or right handed substances, e.g. cane sugar, d-glucose, camphor (in alcohol solution), quartz (R) etc.

(ii)  Laevorotatory or Left handed: Those substances which rotate the plane of polarization in anticlockwise direction are called Laevorotatory  or Left handed substance, e.g. turpentine oil, a - fructose, quartz (L) etc.

Laws of Optical activity

1.    Biot Laws: In 1815 Biot studied the rotation of plane of polarization and proposed the following laws:

(a)  Angle of rotation of the plane of polarization of the polarized light of definite wavelength is directly proportional to the distance l covered by the polarized light in optically active medium (solid, liquid, gas or solution), i.e.,

(b)  The angle of rotation (?) of the plane of polarization of polarized light is directly proportional to the concentration (c) of the solution or vapor, i.e.,

(c)  If ?1, ?2, ?3........ are the angles of rotation of the plane of polarization light by different optically active media, then the resultant angle of rotation of the plane of polarization by the mixture (when they are non-reactive) is algebraic sum of the angle of rotations by different components.

For Dextrorotatory substance + sign is to be taken with angle of rotation whereas for Laevorotatory substances opposite sign is to be taken with angles of rotation.

(d)  Angle of rotation produced by the optically active substance is inversely proportional to the square of the wave length of polarized light used, i.e.,

Thus the angle of rotation of ted color in white plane polarized light is least and for violet color is maximum. Thus if white plane polarized light is passed along the optic axis of the optically active substance like quartz, the transmitted ray gets dispersed into different colors. This phenomenon is called rotator dispersion.

(e)  The angle of rotation of the plane of polarization of polarized light produced by the medium also depends upon the temperature of the medium.

2.    Fresnel's Explanation: Fresnel's explanation of optical rotation is based on the fact that any simple harmonic motion along a straight line can be considered as a resultant of two opposite circular motions of the same frequency. Fresnel made the following assumptions:

(a)   A beam of plane polarized light on entering a crystal along the optic axis is broking up into two circularly polarized vibrations one clockwise and the other anticlockwise.

(b)   In an optically inactive crystal, like calcite, the two circularly polarized vibrations travel with the same angular velocity.

(c)   In an optically active crystal, like quartz, these two circularly polarized vibrations travel with different velocities so that relative phase difference is developed between them as they travel ahead. In case of dextrorotatory substances, the clockwise rotation travels faster while in a laevorotatory substance, the anticlockwise rotation travels faster.

(d)   On emergence, the two circular vibrations recombine to give a plane polarized beam whose plane of polarization has been rotated with respect to that of the incident beam by an angle depending upon the phase difference between them.

Optical Rotation: When PPL passes through certain substances, the plane of polarization of light is rotated about the direction of propagation of light through a certain angle. This phenomenon of rotation of POP of PPL is called optical rotation. This property is known as optical activity and the substace is said to be optically active substance.

The substances rotate POP of PPL in clockwise direction (R.H.) called dextrorotatory and the substance those rotate POP of PPL in anticlock wise (L.H.) is known as Laevorotatory.

This angle of rotation '?' of POP of PPL depends on.

1.    l (length traversed by light in optically active medium)

2.    c (concentration of solution)

3.    If ?1, ?2, ?3 ..... are the angle of rotation of POP of PPL by different optically active media, then the resultant angle of rotation of POP by the mixture is algebraic sum of the angle of rotations by different components.

For Dextrorotatory positive sign and for Laevorotatory negative sign is to be taken in consideration.

4.    It also depends on temperature.