The polarimeter

A polarimeter is an instrument for measuring the angle of rotation of plane polarised light. The unit has an ordinary light bulb at the bottom which is usually powered by a power pack set to the correct voltage. A Polaroid filter converts this ordinary light to plane polarised light which then passes through a test solution in a glass tube. At the top of the unit there is another Polaroid filter within a rotatable knob. Looking down through the filter in the knob, the knob is rotated until the dimmest point is reached. Then the angle of rotation of the plane of the plane polarised light is read of the scale on the knob. If the sample is optically inactive there will be no rotation of the plane of the plane polarised light.

Textbooks sometimes say that the light will become extinguished when you are correctly reading the polarimeter. However, whilst this is fine in theory, a small amount of light always gets through, particularly when the sample tube contains a liquid. So we are looking for the dimmest point when setting the polarimeter. It's easiest to concentrate on the image of the lamp filament which is found at the centre.

Video - the polarimeter