School of Physics Lecture Demonstration Manual

Og-5 The Barbers Pole (optical Activity)

Published: Wednesday 15 March 2006 - Updated: Monday 28 March 2011

Aim

To demonstrate that the rotation of the plane of polarisation in a sugar solution ((optical activity) is wavelength dependent.

Apparatus

Diagrams

Click on pictures to enlarge.

Barbers Pole

Barbers Pole Diagram

Videos

Barbers Pole

Description

A parallel beam of light is reflected into a large beaker containing a sugar solution of 3 kg of sugar in 4 kg of water. The incoming light is first polarised, the polaroid sheet mounted on the spindle of the motor M. As the light traverses the solution, the plane of polarisation is rotated by the action of the sugar molecules. This rotation is wavelength dependent and hence, a spiral of rainbow colours results. As the polariser rotates the spiral rotates.

Using another polariser above the beaker to receive the transmitted light results in a single broadbeam of light whose colour runs right through the spectrum as the polariser is rotated. This beam of light is projected onto the ceiling of the lecture room giving rise to a bright series of transmitted colours.

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