Two pendulums are suspended in two different planes for independent oscillation. Allow the two pendulums to oscillate freely by keeping them away from each other using both of your hands. Use the view finder whose one eye is fitted with red coloured glass to watch the pendulums keeping both of your eyes open.

What did you observe?
The pendulums appear to rotate in three dimension.

The science behind it :
When we are using the view finder, our retina receives more light from plain glass and less light from red filtered glass. This variation creates an illusion of three-dimensional effect on the retina of our eye.