Question

Explain how a polarimeter works and what it measures.

Short answer

Polarimeter works on the principle of rotation of plane-polarized light in order to determine the direction and specific rotation of an optically active substance.

Step-by-step solution

Polarimeter

Polarimetry is used to measure the optical activity of the compounds (a substance is said to be optically active if it rotates the plane of polarized light).

A polarimeter is an optical instrument which measures the rotation of polarized light when it passes through an optically active substance and also measured the tendency of the molecule to rotate plane-polarized light toward the clockwise or anti-clockwise direction.

Working on the polarimeter

The basic principle working of a polarimeter comprises of following combinations: a pair of crossed polarisers used as a polarimeter, light source, sample tube, telescope and analyser.

When the light beam emerging from the source passes through a crossed polariser meaning a pair of polarisers having axes perpendicular, then no light emerges from this combination.

Now, introduce an optically active substance between them. Therefore, the plane of polarisation of light emerging from the source will not be blocked by a second polariser because light emerging from an optically active solution may be rotated by a certain angle ($\alpha$).

The second polariser, also known as the analyser, capable of rotation about the common axis, has to be rotated by an angle in the same sense to make the field of view dark again.

This rotation angle can be measured on a circular scale fitted with the Vernier. After passing through an analyzer, the light, is viewed through a telescope focused on the half shade device.

The formula for the calculation of the angle of rotation or specific rotation is shown below:

Application of polarimeter

A polarimeter is used to measure an optically active substance's direction and specific rotation.

A polarimeter is used to measure the ratio of enantiomers in the solution and also identify which isomer is present in sample

It is also used to determine the specific rotation of given optically substance at different concentrations.