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Thermodynamics

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Optical Activity

When a polarizer and an analyzer are crossed, no light emerges cut of the analyser. When a quartz plate cut with its faces parallel to the optic axis is introduced between N₁ and N₂ such that light falls normally upon the quartz plate, the light.

The quartz plate turns the plane of vibration. The plane polarized light enters the quartz plate and its plane of vibration is gradually rotated as shown in fig.

The amount of rotation through which the plane of vibration is turned depends upon the thickness of the quartz plate and the wavelength of light. The action of turning the plane of vibration occurs inside the body of the plate and not in its surface. This phenomenon or the property of rotating the plane of vibration by certain crystals or substances is known as optical activity and the substance is known as an optically active substance. It has been found that calcite does not produce any changes in the plane of vibration of plane polarized light. Therefore, it is not optically active. Substances like sugar crystals, sugar solution, turpentine, sodium chlorate and cinnabar are optically active. Some of the substances rotate the plane of vibration to the right and they are called dextro-rotatory or right-handed. Right-handed rotation means that when the observer is looking towards light travelling towards him, the plane of vibration is rotated in a clockwise direction. The substances that rotate the plane of vibration to the left (anti-clockwise from the point of view of the observer) are known as laevo-rotatory or left-handed.

It has been found that some quartz crystals are dextro-rotatory while others are laevorotatory. One is the mirror image of the other in their orientation. The rotation of the plane of vibration in a solution depends upon the concentration of the optically active substance in the solution. This helps in finding the amount of cane sugar present in a sample of sugar solution.