Passive 3D
To watch a 3-D movie, you need 3-D glasses that create the illusion of a three-dimensional picture. Passive 3D glasses are one of the two main types of glasses. Passive 3D works using polarized lenses that block out certain wavelengths of light to each eye, creating the illusion of depth and enabling a 3-D look to a movie. Movie theaters showing a 3-D movie provide passive 3D glasses to their customers. Many 3-D TV manufacturers include passive 3-D glasses with the purchase of their 3-D TVs.
There are also active 3-D glasses, which work by use of small LCD (liquid-crystal display) screens that alternate the dimming of the lenses. The TV itself controls dimming by sending an infrared signal to the glasses, telling it which lenses to dim and when. Many people prefer passive 3D to Active 3D due to the cost and weight of the glasses. Passive 3D glasses are much cheaper and lighter than Active 3D glasses. Also, while it's a matter of opinion and dependent on the TV to an extent, some people consider passive 3-D to produce a better image quality versus Active 3D.