How does an unpolarised light gets polarised, when passed through a polaroid? Two polaroids are set in crossed position. A third Polaroid is placed between the two, making an angle 0 with the pass axis of the first Polaroid.
When an unpolarised light beam is incident on a polaroid, then only those vibration of electric vector which are parallel to crystallographic axis of polaroid are transmitted through polaroid and other vibrations are being absorbed by it. This selectiye absorption of electric field vector, which is not parallel to axis is termed as dichroism and hence, plane polarised light is produced by polaroid.