Polaroid glass is used in sun glasses because

A polaroid allows a plane polarised light whose plane of polarisation is parallel to the axis of transmission.An unpolarised light of intensity I0 can be split in to two plane polarised lights of intensity I02, whose planes of polarisation are mutually perpendicular. The polaroid allows the plane polarised light, only whose plane of polarisation is parallel to the axis of transmission of the polaroid. So intensity of emergent ray becomes half.