3D TVs create the illusion of three-dimensional images. Some 3D TVs require electronic 3D glasses. Electronic 3D glasses are usually used with active 3D TVs. They alternately "block" the left and right eye's view, resulting in what feels like a 3D experience. Passive 3D technology uses polarising 3D glasses. The viewer sees the entire image at once, but in each eye, a different line is shown. This creates the impression of a 3D image.