Why does unpolarised light from a source show a variation in intensity when viewed through a polaroid which is rotated?
A polaroid consists of long-chain molecules aligned in a particular direction. The electric vectors (associated with the propagating light wave) along the direction of the aligned molecules get absorbed. Thus, if an unpolarized light wave is an incident on such a Polaroid, the light wave will get linearly polarised with the electric vector oscillating along a direction known as the pass-axis of the polaroid, perpendicular to the aligned molecules. Therefore, if the light from an ordinary source passes through a polaroid sheet, its intensity is reduced by half. Rotating the Polaroid has no effect on the transmitted beam, and the transmitted intensity remains constant.