A lithographic apparatus includes an illumination system, support constructed to support patterning device, a projection system, an interferometric sensor and a detector. The interferometric sensor is designed to measure one or more wavefronts of a radiation beam projected by the projection system from an adjustable polarizer. The interferometric sensor includes a diffractive element disposed at a level of a substrate in the lithographic apparatus and a detector spaced apart from the diffractive element, the diffractive element being arranged to provide shearing interferometry between at least two wavefronts mutually displaced in a direction of shear. The detector is designed to determine, from the wavefront measurements, information on polarization affecting properties of the projection system.