Final answer:
3D movies use the depth cue of stereopsis to create a three-dimensional effect on a flat screen, which is enhanced through the use of 3D glasses that filter images differently for each eye.
Step-by-step explanation:
3-D movies rely on the monocular depth cue known as stereopsis to create the illusion of depth on a flat movie screen. Stereopsis is a binocular cue that involves the slightly different images that our two eyes perceive due to their lateral separation. When the brain combines the two images, it creates the perception of three dimensions. 3-D glasses enhance this effect by filtering the projected images differently for each eye, thereby tricking the brain into perceiving depth where there is none. Without these glasses, both images would appear blurry and superimposed, thus collapsing the depth effect. However, this question also tests the understanding of monocular cues and their role in depth perception, indicating interposition as a monocular depth cue but not the main answer in this context.