In a darkened classroom, demonstrate the properties of a reflection and a transmission hologram:

1. Use a point source of incandescent light—such as a “spotlight” available in hardware stores—and illuminate a typical white-light reflection hologram. Demonstrate the full parallax of the virtual image by allowing students to look up and down and left to right.

   Now hold the emulsion side of the hologram on top of a cup of hot water (tea, coffee) and allow the steam to “swell” the gelatin. Observe the image again and notice that the color has been shifted toward red. This demonstrates that reflection holograms, although recorded on “black-and-white” photographic emulsion, can recreate three-dimensional images in color. This shows the essence of Lippmann photography (Nobel Prize in physics, 1908) and Bragg diffraction (Nobel Prize in physics, 1915).

2. Use a two-channel transmission hologram. Project a real image onto a screen using a collimated beam of laser light (or a laser pointer). Move the laser beam around the hologram and show that each spot recreates a distinctly different view of the object. Thus, if the hologram were broken into pieces, each piece would show a complete view of the recorded object.

   Now change the angle of incidence on the same hologram and note that a completely different view appears—with all the same characteristics of the first image.

   With use of a diode laser without the collimator, the diverging light can now be used to show the virtual image of each of the two channels in the hologram. Students must come up individually and look through the hologram as if it were a window with the object(s) on the other side. Point out that this demonstrates the enormous information-storage capability of holograms.