This is a set of 25 slides of classic optical illusions, including: impossible figures, oscillating figures, size illusions, perception illusions, and convergence/divergence illusions.

The demonstrator opens and closes iris diaphragms in front of white lights out of phase with respect to each other (e. g. one is closed while the other is opened, and vice versa).

Observers experience the illusion that the white spots are moving toward and away from them, rather than changing in size. This one must be done in a room darkened as much as possible to remove visual reference points.

There are two sides to the gizmo shown in the photo above - the front side has no "spot." When you hold that side up and rotate it rapidly in about an 8-inch diameter, spots similar to the one in the photograph "appear" due to "rinsing" by the eye.

The gray circles appear due to the process of "rinsing," which is a type of stroboscopic effect between the pinwheel and your eye motions. These things with a large number of different designs are used as research tools in investigating eye motions. What you see depends on the design, how fast you move it in circles, and your eye motions. Clever researchers have produced big books of hundreds of these designs for which the particular pattern seen depends on the nature of the eye motions, allowing them to deduce information about eye motions.

SUGGESTIONS: With a bit of practice you can start rotating it with the non-spot side up, then flip it after the spots start to appear, magically creating a spot where none previously existed. This can be an amusing magic trick, but be sure to explain the real mechanism afterwards!

Electronic versions of the pinwheel blanks and instructions available upon request; email lecdemhelp at physics.

The trapezoidal window is rotated at about one revolution per two seconds by an electric motor in the box. When the taller side rotates away from us, it appears to reverse direction and rotate toward us.

Everyone knows that any object appears to get smaller as it moves away and bigger as it moves toward you. Mind over matter!

The classic impossible triangle is constructed from three orthogonal aluminum bars. The front bar has a cutout which is aligned with the rear bar so that the triangle appears closed when viewed from a particular point (or along a short line), forming an impossible figure.

This gizmo bothers people more when it is set up with a pendulum apparently swinging through the "solid" metal of the triangle. The photograph shows the triangle with the pendulum at its equilibrium position along with the view of the triangle from an appropriately positioned camera.

Photographs of actual 3D realizations of a number of Escher art works will be found at the "Escher for Real" web site http://gershonelber.org/EscherForReal/.

Note: Due to complexities of setup, please give at least three working days notice when ordering this demonstration.

Due to the complexity of setup, please give a minimum of three working days notice when ordering this demonstration.

This demonstration is primarily intended for special events rather than in the classroom. For routine classroom use, consider the tabletop version O4-34.

A double bright point source with red and blue filters, as shown in the drawing below, is used to shadow a crystal model from behind on the lecture hall rear projection screen. Observers in the lecture hall wearing red/blue 3D goggles see apparent three dimensional motion of the crystal when the crystal is either rotated or moved back and forth between the sources and the screen. The shadow of the red light can be readily seen in the photograph, but the blue shadow is more difficult to observe.

A slick variation can be carried out as follows: Shadow your hand in the lights as you pretend to grab at the projectors. Your hand will appear to the group to get very large and grab at them! This gets a great response every time.

This technique has been used for 3D TV movies. We have an example of this, a monster movie, in our video file.