PURPOSE: Demonstrate visual latency and the Pulfrich phenomenon in a very dramatic way.
DESCRIPTION: Stand back 8-10 feet or more from the pendulum and swing the pendulum perpendicular to your line of sight. Watch the bob with both eyes and hold a dark filter over one eye to see the pendulum appear to move in an elliptical path. Hold the filter over the other eye to make the direction of rotation reverse. This effect is due to a delay in synapses along the optical nervous system between the retina and the brain of dimmer light signals relative to bright signals. Any dark filter or polaroid can be used; we use either the red or the blue filter from a pair of red/blue stereoscopic goggles. In darker rooms it may help to illuminate the pendulum bob with a goose neck lamp.
This is just like Demonstration O2-11: PULFRICH PENDULUM, except that the pendulum has a bifilar suspension. Some people like this one better because the pendulum never changes plane, and some people like the other one better because the bifilar suspension gives additional optical clues to observers at an angle with respect to the two suspension strings.
REFERENCES: (PIRA unknown.) See Demonstration Reference File for lots of information on various aspects of the Pulfrich phenomenon and visual latency, as well as a nice detailed explanation of the Pulfrich pendulum.
EQUIPMENT: Pendulum with white bob, goose neck lamp with 100 watt bulb, dark filters.
SETUP TIME: None.
Go back to Lecture-Demonstration Home Page.