Detection of plane polarised light: Naked eyes or the polarizer alone cannot make the distinction between unpolarised light and plane polarised light. To analyse the nature of light, another crystal (analyser) is used. The tourmaline crystal is used to produce plane polarised light.
If the polarizers rotated in the path of the ordinary light, the intensity of the light transmitted from the polarizer remains unchanged. It is because, in each orientation of the polarizer, the plane polarised light is obtained, which has vibrations in a direction parallel to the axis of the crystal in that orientation. If the analyser is rotated in the path of the light transmitted from the polaeizer, so that the axis of the polaeizer and the analyser are parallel to each other, then the intensity of light is found to remain unaffected.
If the axis of the polarizer and the analyser are perpendicular to each other as shown in the figure, then the intensity of light becomes minimum.
In this position, the polarizer and the analyser are said to be in crossed position.
