PURPOSE: To observe an almost complete circular rainbow formed by glass beads.

DESCRIPTION: Glass beads are glued to a black screen to simulate water drops. When the screen is viewed from close up, with the light source (or sun) coming from behind the observer, the primary rainbow can be seen at an angle of about 22 degrees. The secondary rainbow is at an angle of about 88.5 degrees, and must be viewed by looking nearly parallel to the surface of the card, so it cannot be seen while viewing the first order rainbow. This is best observed on an individual basis. The dark region in the center of the photograph is a shadow of the camera taking the photograph, where the observer would position his or her head.

Several features of this rainbow are similar to features of real rainbows: the colors are in the correct order (red outside and blue inside, a bit washed out in the central region in the photo) and are reasonably realistic, the area outside the rainbow is very dark, compared to the area inside the rainbow, and several supernumerary bows (white circles) can easily be seen inside the rainbow.

The real primary rainbow is at about 41 degrees, and the secondary rainbow is at about 52 degrees. The higher index of refraction of the glass beads shifts the primary rainbow to 22 degrees and the secondary rainbow to 88.5 degrees.

SUGGESTIONS: The picture above was taken by a 35mm camera using a 20mm wide-angle lens. The camera was close to the glass bead screen along the line between the point source and the center of the screen. Note that the full angle of the rainbow in this case is about 45 degrees, whereas the full angle of a real rainbow is nearly 90 degrees.

Click your mouse here to see complete instructions for how we make the glass bead rainbow.

Below are photographs of two very large "human rainbows." Your task is to comment on the accuracy of these rainbow representations.

Click your mouse here to see the world's largest human rainbow, created by over 31,000 students, faculty, staff, and alumni at the Polytechnic University of the Philippines.

Click your mouse here to see the human rainbow created at the celebration of the 65th anniversary of The Hong Kong Polytechnic University.

NOTE: the two "rainbows" above are incorrect in that they have put a magenta color at the blue end of the rainbow. This end of the rainbow should be pure dark blue; it cannot contain red (to form magenta) because red is at the other end of the spectrum, as seen in the photograph of the "fire rainbow" below.

The photograph below shows a phenomenon known as the "fire rainbow." The photograph and related text has been taken from http://www.snopes.com/photos/natural/firerainbow.asp Urban Legends.

This is a real photograph of an atmospheric phenomenon known as a circumhorizon(tal) arc, the example shown above was captured on camera as it hung for about an hour across a several-hundred square mile area of sky above northern Idaho (near the Washington border) on 3 June 2006.

In general, a circumhorizontal arc (or "fire rainbow") appears when the sun is high in the sky (i.e., higher than 58° above the horizon), and its light passes through diaphanous, high-altitude cirrus clouds made up of hexagonal plate crystals. Sunlight entering the crystals' vertical side faces and leaving through their bottom faces is refracted (as through a prism) and separated into an array of visible colors. When the plate crystals in cirrus clouds are aligned optimally (i.e., with their faces parallel to the ground), the resulting display is a brilliant spectrum of colors reminiscent of a rainbow.

REFERENCES: (PIRA 6A46.16) See Demonstration Reference File for several interesting papers covering aspects of the rainbow and this demonstration.

EQUIPMENT: Black screen with 0.007" diameter glass beads, bright point source or sunlight.

SETUP TIME: None.