This demonstration explains the polarisation of light. Two polaroids are placed on an overhead projector; light transmitted through the first polaroid becomes polarised, and by rotating a second polaroid through 360 degrees the effect of polarisation can be seen as the transmitted light becomes alternately brighter (when the polaroids are aligned) and dimmer (when they are crossed).

When the lower polaroid's plane of polarisation is parallel to the second polaroid, all the light is transmitted brightly through both filters. By rotating the angle of polarisation, the planes between the two polaroids can be crossed so that they become dark, with no light transmitted.

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