This museum piece is a polarimeter. Polarimeters are used to measure the type and change of angle of polarisation that light undergoes when passed through material. It is known that light travels as both a particle (a photon) and a wave, and polarisation occurs when the wave of light is specially oriented. When light propagates in only one plane (only up-and-down, or only left-and-right) it is said to be linearly polarised. It is linear polarisation that this polarimeter tests for.
When using a polarimeter, a light source is set up at one end, and is shone through an initial light polarizer to give the incoming light a known polarisation. This initial polarisation allows the user of the polarimeter to test the sample for set properties. For example, the sample being tested could rotate the angle of linear polarisation. Once the light has passed through the sample, it enters the eyepiece. This eyepiece is attached to a polarising plate, and by rotating the eyepiece until maximum brightness is observed, the user can find the angle of polarisation by reading it on the round metal plate marked in angle increments attached to the eyepiece.
The eyepiece consists of a telescope with an infinite focal length so as to ensure a parallel beam, and is rotatable via a small dial located at the base of the metal plate. This polarimeter is best suited for analysing liquids that are capable of only weakly changing the angle of polarisation. This is due to the initial polariser being able to be set to a range of -20° to +20°, and the telescopic eyepiece only being able to rotate very slowly. This would make the polarimeter ideal for experiments like testing the amount of sugar dissolved in water, as sugar-water rotates the plane of linearly polarised light.
This polarimeter was manufactured by W. Wilson in London, likely in the late 19th or early 20th century. It consists of a cast-iron base with three legs, and brass casings for the optical sections, such as the telescopic eyepiece. The polarisers built into the polarimeter cannot be removed, and are polarising prisms, likely Nicol prisms. It should be noted that the centre section of the device where the liquid sample would be held is missing.
This item is part of the UQ Physics Museum ‘Before Photonics’ Tour
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