Engagement Suggestion

• • The box is first shown to the group. Then the black cloth is placed over the front of the box, the trap door on top of the box opened, and the rabbit put into the box through the trap door.
• • Invite students to predict what they will see when the cloth is removed.
• • When the black cloth is removed the rabbit has vanished into thin air (behind the mirror).
• • Challenge then to analyze how this has happened
• • Explain the positioning of the mirror, and invite them to consider what it would look like with the mirror at different angles.

Background

The large box has holes in each end for people to insert their heads, and a half-silvered mirror in the middle. A special variac has been wired so that when the knob is turned lights in each end of the box change intensity with one getting brighter while the other gets dimmer, and vice versa. Each viewer will see a metamorphosis of his or her face to that of the person on the other side. This device was made by an art student as a class project and donated to the Lecture-Demonstration Facility.

A second hole in one end allows the action to be viewed by a video camera and displayed for the entire class, as seen in the photographs below.

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.

Students can stand in front of the concave side at different distances to find the focal point. Invite students to predict the orientation of the image they see at different points, then try it out.

Invite different students to view the tank from different angles and draw what the rod looks like. Have them compare their experiences and discuss.

A video camera is optionally available to make this more visible in large lecture halls.

Background

Collimated light waves come from the laser and pass through a pair of narrow slits in the slide; the light passes through and then projects on the distant screen. But light travels as an electromagnetic wave, so when the light comes out of the two slits, it forms two wavefronts, just like ripples from two stones dropped in a pond. These two wavefronts can interfere with each other, as we can model with this pair of overlapping concentric circles. Where two peaks or two valleys of the wave pattern line up, they add together, interfering constructively; when a peak and a valley overlap, they cancel out, interfering destructively. The same happens with light waves; the light from the two slits overlaps, and creates a pattern of bright spots (constructive interference) and dark spots (destructive interference). The spacing between the bright and dark fringes ultimately depends on three things: the distance between the slits and the screen, the wavelength of the light, and the spacing between the two slits.

This is one of the keystone experiments in establishing light as a wave (rather than particles). The laser beam is expanded to around 4 cm diameter and passed around a 1 inch ball bearing which is suspended between two pointed rods. The diffraction pattern with its central bright spot is viewed on a distant screen. In the photograph at the left above the alignment of the ball in the expanded laser beam can be seen on the screen. In the photograph at the right the Poisson bright spot is seen on a ground glass screen about twenty feet from the laser, looking back toward the laser beam. This picture can be displayed on a video monitor or using a video projector.

There is a fascinating story about the origin of this experiment, referenced from Eugene Hecht, Optics (Second Edition) and a the web site Fresnel Diffraction, written by Dean Dauger:

In 1818, Augustin Fresnel submitted a paper on the theory of diffraction for a competition sponsored by the French Academy. His theory represented light as a wave, as opposed to a bombardment of hard little particles, which was the subject of a debate that lasted since Newton's day. Siméon Poisson, a member of the judging committee for the competition, was very critical of the wave theory of light. Using Fresnel's theory, Poisson deduced the seemingly absurd prediction that a bright spot should appear behind a circular obstruction, a prediction he felt was the last nail in the coffin for Fresnel's theory. However, Dominique Arago, another member of the judging committee, almost immediately verified the spot experimentally. Fresnel won the competition, and, although it may be more appropriate to call it "the Spot of Arago," the spot goes down in history with the name "Poisson's bright spot" like a curse.

Note that the alignment of this system can be delicate and time-consuming; it is not recommended to combine with with other demonstrations using the same laser in a single 50-minute lecture.