Step 2: Experiment with Scratch Holograms
Try closing one eye and holding a finger next to one spot of light. Now close the other eye and notice the distance between the spot and your finger. Did you notice that the spots on the larger scratches are further apart? These two spots are the "Stereo Pairs" that are creating the 3D image.
If you saw the spot in the same location with both eyes, it would appear to be on the surface of the plastic. Your brain uses the differences between the two images from each eye to judge depth. Have you ever held your finger in front of you and noticed how its position seems to change as you blink your eyes?