What it shows:
A beam of white light incident on a giant raindrop (simulated by a water-filled round flask) produces a full rainbow of colors. As with real rainbows, one can also see that the light intensity inside the rainbow is much greater than outside the rainbow.
How it works:
A Florence (round-bottomed) flask is completely filled with water and sealed with a rubber stopper. A Beseler slide projector 1 serves as the sunlight. The light incident on the giant raindrop is refracted, reflected, and refracted once more, back in the direction of the slide projector, in a 42° cone of light. The cone of light falls on a large screen (behind the slide projector), producing a rainbow.
The primary rainbow is created by light that undergoes one internal reflection inside the raindrop. Ordinarily, this is not a total internal reflection and most of the light passes on in the forward direction. So as not to "waste" this light, we have silvered half of the flask so that all of the light is internally reflected and contributes to the rainbow. This makes for a more intense rainbow and, although a little bit of a cheat, is visually more satisfying.
Setting it up:
The slide projector sits on the lecture bench and is aimed towards the feet of the front row seats. The circular aperture slide produces a confined beam of light—otherwise the light would shine into the audience. Lower the large front projection screen all the way down. The lecturer holds the water-filled flask (by it's long neck) in the beam of light.
Newton was the first to explain the colors of the rainbow. Descartes understood the intensity distribution in the cone of light but was unaware of dispersion effects. Placing a color filter in front of the slide projector will produce a monochromatic rainbow—Descartes' rainbow.
Don't expect the colors of the rainbow to be pure—they're not. Red is the rainbow's purest color since no other color of light comes out of the drop at 42°. All the other colors overlap each other. For example, at 41°, green light is within the red cone of light and so has some red mixed into it. At 40°, blue light has light of all colors mixed in.
A 3" (7.6 cm) diameter acrylic ball can also be held in the beam of light to demonstrate how the angular size of the rainbow depends on the index of refraction. The plastic rainbow is noticeably smaller.
1 Slide King II, Charles Beseler Co., East Orange, NJ. A "slide" (made from 1/16" thick aluminum sheet cut to the standard 3.25" × 4" size) with a 2" diameter hole in the middle produces a spotlight-like beam.