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Rainbow Demonstration

It is difficult to demonstrate a rainbow in a classroom and almost impossible to get the brilliance of one seen in the open air. The main reason is that only a small fraction of the light passing through a water droplet (or its simulation) is reflected into the rainbow ray. Although the rainbow angle can be demonstrated with a laser, the full beauty of the colors comes only when a white light source (containing all these colors) is used. Sunlight is best, of course, but a bright incandescent light whose radiation is made to simulate sunlight (parallel rays) can work. There are two examples given here that can produce the rainbow pattern, and Dr. Tom Arny suggests others.

This illustration is adapted from Minnaert Light and Colour in the Open air

If sunlight is allowed to shine through a central hole in a piece of white cardboard and illuminate a spherical flask, a faint rainbow will appear on the cardboard. It has the shape of a closed circle and its angular distance is about 42 degrees, with red on the outside, as in a real rainbow. If the back side of the flask is painted with silver or aluminium paint, more of the light will be reflected and the rainbow should be brighter.

The second experiment is taken from Bohren Clouds in a glass of Beer .

He suggests using a water-filled beaker that has a wrapping of a flexible mirror made from aluminized plastic film covering the back side so that no light is transmitted through the beaker. The light comes from a slide projector in which an opaque slide with a slit in it is inserted. The rainbow is seen on the screen. Bohren points out that it is bright on the inside of the bow from the refracted light inside the rarinbow angle.

Dr. Tom Arny, University of Massachusetts, has mentioned some of the items he uses to illustrate rainbows in his classroom laboratory. He writes

You can buy plastic spheres that illustrate how a rainbow forms fairly well.

Our physics demonstration laboratory also has some tiny glass beads from a reflective paint manufacturer that create a reasonable rainbow.

For classroom use, although a bit messy, you might get a small squirt plant mister. I've also used microscope slides glued edge to edge and stuck in plastacine to make a model ice crystal. Other demos are the simple pencil in a glass of water to show bending and light coming into the top of an aquarium and throwing a spectrum on the floor.