What solid object has circles as its front and side views, as shown below, but its top view is not a circle?

This can also be useful when discussion ideas of perspective and imaging.

This can be useful when discussion ideas of perspective and imaging.

Plastic frame with two colored rods. The trick here is to determine, solely by looking at the frame from two sides set 90^o apart (as seen above) whether the sticks are touching or not.

This can be useful when discussion ideas of perspective and imaging. Try showing the model from a single angle, perhaps with a video camera, and invite students to draw it; then rotate it and have them compare their results.

These spotlights use dichroic filters to achieve their color.

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.

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.

Opening the box and viewing the system from above, it appears to be only a random collection of sticks. When viewed through the peephole at the front end, the sticks appear to form a chair.

This illusion and many more like it were made by twentieth century Dutch artists who didn't have enough to do, forming an entire branch of modern art: the "perspective cabinet."

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.

Polaroids at +&- 45 degree diagonal orientation positioned in front of the double point source shadow project a crystal model rotating on a 33 1/3 RPM turntable onto an aluminum sheet. Viewers wearing polarized 3D glasses see the a ction in three dimensions.

This technique is used in many really high-class three-dimensional movies, such as that at Disney World in Orlando. The screen must be a conductor so that the polarization of the reflected wave is maintained.