Here is a great optical illusion one can easily perform with a pendulum.
Watch the (real world) pendulum swinging while sitting perpendicular to its motion (as you would normally watch a pendulum) while you and the pendulum are in a brightly lit room.
Cover one eye with a dark sunglass lens. The pendulum will appear to swing in an oval. Cover the other eye with the sunglass lens. The pendulum will appear to swing in an oval in the opposite direction.
The illusion appears to indicate that our vision system uses a timing delta of a moving object as seen from one eye and another to help it as part of several visual clues to determine 3D distance from the viewer. Couple this with the fact that our vision system processes information faster under bright lighting and slower under dim lighting. The eye with the sunglass lens reports the motion delayed and so the brain calculates a different distance as the pendulum moves at different speeds.
Cheers,
Darius