The teleidoscope is a kaleidoscope with a telescopic eye. Rather than using bits of colored glass, the teleidoscope projects the light rays down into the tube and mirrors create multiple reflections. Each reflection forms one section of the pattern.

For the teleidoscopic effect move the lamp, not the teleidoscope. Or, move objects in front of the lamp such as a printed page. Set lamp on LOW. CAUTION: Do NOT let the light shine directly into the TV camera.

The kaleidoscope consists of three 4'x6' mirrors forming an equilateral triangle about three feet off the floor. Several series of reflections can be observed as the participant moves about the inside of the kaleidoscope.

Shown below are a few pictures of Gwen having fun in the Kaleidoscope. If Gwen can have this much fun alone, just think of how much fun can be had by a bunch of people in there together!

This was more or less in permanent residence in the Physics Lecture Hall Lobby for several years, but has been retired for the time being.

This is a long focal length concave parabolic mirror that produces an image between the object and the focal point of the mirror for the setup photographed. The central ray passes through a half-silvered mirror which deflects half of the light to the tip of the object arrow. The upper mirror can be rotated to create any of the three principal rays. A convex lens at the left keeps the light ray narrow.

The object can be moved within the focal point of the mirror to produce an enlarged virtual image behind the mirror.

Spherical aberration can be shown using a number of rays to show how they focus. The number and spacing of rays can be changed by choice of slit baffle and changing the position of the baffle. A baffle with pairs of rays equidistant from the optical axis of different colors can be used to emphasize the difference between pairs of rays.

Using a rectangular open baffle the caustic can be illustrated, as in the photograph at the right above.

A container of glycerine darkened by blue food coloring is rotated at 45 RPM so that it forms a parabolic mirror with a focal length of about 22 cm. A lighted object 70 cm above the surface forms a virtual erect image 70 cm below the surface when the liquid is quiescent, and a real, inverted image 32 cm above the surface when the liquid is rotating. A ruler is positioned at the height of the real image for reference.

The image can be viewed directly individually from above or it can be shown to the entire class using the TV camera and monitor. The difference in the focus for the two images is seen very nicely; the real image is clearly in focus with the ruler, while the virtual image is both inverted and out of focus. Below are views of the image as seen by the video camera mounted next to the object cross: image of cross with liquid at rest (second left), camera focused on ruler with liquid at rest (center), and image with liquid rotating, showing image and ruler in focus at the same point (right).

Many modern telescope mirrors up to 25 feet in diameter are constructed using this technique of "spin-casting." A container of molten glass is rotated as it cools, forming a nearly perfect parabolic mirror, the surface of which is then ground to eliminate small errors and coated with a reflecting material.

This demonstration uses one of the old overhead transparency projectors that focuses the light by a large parabolic mirror under the platform (rather than a Fresnel lens on the platform as in newer models), as seen in the images above. The heat filter and the mirror system above the projector have both been removed. There is sufficient heat focused about two feet above the projector to burn a piece of black paper in a few seconds. In a dark room, the focal point can be clearly seen as the smoke from the paper scatters the light.

Engagement Suggestions

Invite students to predict what would happen if you used white paper rather than black.

• • Would it still burn?
• • Would it take more or less time to ignite?

Background

This demonstration illustrates two important points. It clearly shows that light can be focused to a point by a curved reflector. It is also an illustration of infrared radiation, and the connection between light and heat. When appropriate to the course, consider combining this with a discussion of the wavelengths of the electromagnetic spectrum, and the relationships of energy, heat, and temperature.

Invite students to place their hand through the image, to get a feel (or not) for what's happening. Have them consider where such images might be used.

The laser beam is directed onto the image of the penny from demonstration L3-19: PENNY AND PARABOLIC MIRRORS. It appears to be reflecting from the image, although the multiple reflections seem to disperse the laser beam so that it does not emerge intact.

A little fog from the fog machine shows the incoming laser beam. Notice that because laser light is more dense than air some of it remains in the illusion chamber after the laser is turned off (photograph at right).