In 3D movies, two images are projected simultaneously on the screen. One image is horizontally polarised and the other is vertically polarised. That is why, without 3D glasses, the image is blurred. The 3D spectacles that we use essentially have two types of polaroid glasses, horizontally polarised for, say, the right eye and vertically polarised for the left eye. When the reflected image from the screen falls on the spectacles, the right side glass allows only the horizontally polarised component to pass and blocks the other component while the left glass only allows vertically polarised light. So, each eye gets an independent view. However, the brain combines these images and interprets the combination as a realistic 3D image. Of late, the linearly polarised glasses are replaced by a set of circularly polarised glasses that can provide a 3D effect for the viewers even if they tilt their head.
REFERENCE:
APRIL 01, 2017.Why do we wear glasses to view 3D movies?.The Hindu.retrieved from
http://www.thehindu.com/sci-tech/science/why-do-we-wear-glasses-to-view-3d-movies/article17757804.ece
