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Substance are optically in the sense that the rotate on incident plane or polarized light so that the transmitted light energy at a measurable angle to the plane of the incident light.

Plane polarized light: 
Plane polarized light

According to wave theory of light, an ordinary ray of light is considered to be vibrating to all planes at right angle to the direction of its propagation.If this ordinary ray of light is pass through a Nicol prism the emergent ray has its vibrations only in one plane.This light having wave motion in only one plane is known as plane polarized light.

Optical Activity:
Optical rotation & construction of polarimeter

When certain organic liquid solution (like sugar) or quartz crystal are placed in the path of plane polarized light, the plane of polarization is rotated.The property by virtue of which the plane of polarization of light is rotated is called optical activity and the substances possessing this property are said to be optically active. Substances which rotate the plane of polarized light toward the right (clockwise) are called dextro rotation (+) while which rotated toward left (anticlockwise) are called laeve rotation (-) .
It has been found that the magnitude of the rotation depend upon the following factors.
  1. Nature of substance.
  2. Length of the liquid column through which light passes.
  3. Concentration of the solution.
  4. Nature of the solvent.
  5. Temperature of the solution.
  6. Wavelength of the light used.     

Polarimeter Principal: 

The optical rotation of a substance is the angle through which the plan of polarization is rotated when polarized light passes through the substance, if liquid or a solution of the substance.
Substance are described as Dextro-rotation (+) or Laevo-rotation (-) according to whether the plan of polarization is rotated clockwise or anticlockwise, respectively as determine by viewing towards the light source.
Specific optical rotation is expressed in degree milliters per decimeter gram.
Calculation the specific optical rotation.

For Liquid [α]²⁵ = α/ld
For Solids [α]²⁵ = 100 α/lc
α = Corrected observed rotation in degrees, at 20°
D = Line of sodium light (589.3 nm)
L = Length of polarimeter tube in dm.
c = Concentration in % w/v

Calibration of polarimeter:

Sucrose used in the calibration of polarimeter previously dried 2 hrs at 105°C and temp. maintaining 25°C ± 0.5°C
Concentration of  Sucrose (%)
Angle of Rotation at (Standard)
25°C ± 0.5°C
Angle of Rotation at (Observed)
25°C ± 0.5°C

13.18 to 13.40

26.36 to 26.80

39.54 to 40.20

52.72 to 53.60

65.90 to 67.00


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