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.

SUGGESTIONS: Click here to see a drawing that helps to explain the situation.

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.

A web site at the Southern Illinois University, Carbondale, The Pulfrich Effect, gives a rather complete discussion of the Pulfrich phenomenon from a historical perspective. The article, Effect of Target Velocity in a Frontal Plane On Binocular Spatial Localization at Photopic Retinal Illuminance Levels, Journal of the Optical Society of America, 1960, 50(10), pp. 970-973 suggests that the effect is due to the photopic retinal illuminance level, not intensity related delays in signal processing at various synapses.

Another very clever visual latency demonstration is Frisen's Lazy Shadow. A central light polygon with a darker outer "shadow polygon" rotates at a constant angular speed, but the darker polygon is delayed in phase due to visual latency. You can vary the parameters of the rotating figure to investigate your visual latency.

EQUIPMENT: Pendulum with white bob, goose neck lamp with 100 watt bulb, dark filters.

SETUP TIME: 15 minutes.

Lecture-Demonstration Home Page