éBack to the Table of Contents

The collection presently includes the following exhibits:

        The Rainbow Page (The Washington Times, November 30, 1996, Page B8)
        The Catalog of Educational Videocassettes for School and Libraries by RAINBOW EDUCATIONAL VIDEO
        RAIN-BLO Assorted Bubble Gum Balls
        Fruit Skittles
        Nestle Rainbow Morsels
        Rainbow Nerds
        Lucky Charms Oat Cereal
        Keebler Bite Size Rainbow Chips Deluxe
        Rainbow-Block GALT TOYS
        Weatherslam Game

All of them show rainbows with colors either reversed or randomly arranged.

Dr. James Watson Jr., who generously contributed to the collection, made the following comments: "My wife and I do "Physics of Toys". I show the stacked colored ring toy and ask about the order. Most say the rainbow order is variable.

My contention is that more is taught outside formal education settings by things like this. I survey my classes each semester and about 25% don't know which color is on top of a rainbow.

My story with the "Rainbow Video" is a good example. At a NSTA meeting several years ago, I saw the cover of the reversed rainbow. I pointed it out in a conversation with my wife. Trying to be nice, I said, "Maybe it's a secondary rainbow". The salesman overheard me and said: "No, we only deal with elementary, not secondary!". Meaning schools! He had no idea what was wrong."

éBack to the Table of Contents

Question to Marilyn: "I've seen double rainbows. Is there such a thing as a triple rainbow?"

Marylin's answer: "Yes, triple rainbows exist. Since you have seen double rainbows, you know that the second one arches over the first one, and the colors are reversed. With a triple rainbow, the third one arches over the second one, and the colors are reversed again - back in their familiar order."

Physicist's comments: Unfortunately, you'll never see outdoors a triple rainbow described by Marilyn. But three, four, five, six or even more rainbows have been occasionally observed. So where's the difference?

Let's begin with a common rainbow. When a beam of sunlight, which contains all colors, enters a raindrop at an angle, the sunlight is separated into a spectrum of colors, each traveling in slightly different direction than the incident beam.  That spreading of light into its color components is called dispersion. After single reflection from the back surface of the droplet, the different color components again change the direction as they re-enter the air, and spread even more, giving rise to observed rainbow. Since the light was reflected only once inside the raindrop, the rainbow is called a primary (k=1) rainbow. However, while most of the light re-enters the air, a remainder undergoes a second reflection inside the raindrop before re-entering the air and forming a secondary (k=2) rainbow occasionally visible above the primary rainbow. The rainbows would be completely round if the ground were not in the way (seen from an airplane near mid-day, the bows form complete concentric circles.)

Since a fraction of light always undergoes additional internal reflection instead of re-entering the air, a single drop of water is theoretically capable of giving an infinite number of rainbows. But the higher the order, the fainter the rainbow. Now, rainbows up to k=13 (i.e. with light reflecting 13 times inside the drop before re-entering the air) have been observed from a single drop of water suspended from the end of a wire in the lab. And rainbows up to k=19 have been observed in thin, falling streams of water, again in the lab.

But the rainbows with k=3 (Marilyn's triple rainbow) and beyond are observed only in lab. To my knowledge they have never been observed outdoors. Here's why:

You'd see the primary and secondary rainbow only if the Sun is behind your. But the third (k=3) and fourth (k=4) rainbows are formed behind your back, the fourth just above the third. You'll never see them, as they are much fainter than the first two, and you'd have to look in the direction of the Sun, where a glare from the sunlight will make an observation impossible. The fifth (k=5) and sixth (k=6) rainbows form again in front of you, the fifth just below the secondary and partially overlapping it, and the sixth below the primary. But again, you'll never observe them outdoors, since the fifth one is only 10% as bright as the primary, and the sixth only 8% as bright as the primary. Note that the relatively common secondary rainbow (k=2) is about 43% as bright as the primary, yet it is hard to see it most of the time. See the excellent paper by Jearl D. Walker, American Journal of Physics, Vol. 44, No. 5 (May 1976), pp. 421-433 for details of theory and experiment.

I'd like to mention here another beautiful and striking rainbow phenomenon. If you are very lucky, you might happen to witness several faint rainbows on the inner side of the regular primary rainbow (k=1), and if you are extremely lucky, also inside the secondary (k=2) rainbow. They are slightly detached and have pastel color bands that do not fit the usual pattern. They are known as supernumerary rainbows, and their very existence was historically a first indication of the wave nature of light. Some supernumerary rainbows are shown here:

http://www.jal.cc.il.us/~mikolajsawicki/rainbows.htm

http://epod.usra.edu/blog/2002/07/multiple-bows-from-a-water-hose.html

So if you are very, very lucky, you might indeed see several rainbows - the primary rainbow and its supernumeraries, and the secondary rainbow and its supernumeraries.

All rainbows discussed so far are produced when raindrops are illuminated with a light from a single light source - the Sun. But sometimes raindrops are illuminated by a light from more than one source, and might produce corresponding rainbows for each of these sources. What happens under such circumstances is shown here:

http://lapinkavijat.rovaniemi.fi/maupertuis/rainbow.html

This historical  illustration drawn on 27th July 1736 by Pierre-Louis Moreau de Maupertuis during his expedition to Lapland shows 3 rainbows, but not a triple rainbow.  Two concentric rainbows are the primary and secondary rainbows produced by a direct sunshine reflected inside raindrops. But the center of the third rainbow does not coincide with the center of the other two. This is a strong indication that the sunlight was probably reflected from a layer of low clouds or fog, and therefore entered raindrops from a different direction, producing a third rainbow that in my opinion is a primary rainbow corresponding to this additional light source.

And today, in addition to de Maupertuis' drawing, we are also able to enjoy full color pictures of such rainbows:

http://www.flickr.com/photos/billadler/286805940

http://epod.usra.edu/blog/2005/11/multiple-rainbows.html

Thomas David Kehoe reported observation of a similar triple rainbow on the evening of Thursday, June 24, 2005 from the NCAR on Table Mesa in Boulder, Colorado. "There were two primary rainbows, equally bright, one on top of the other, with about one rainbow's space between them. The two rainbows joined together as they approached the ground. The secondary rainbow was in its usual place, with the colors reversed. My guess was that there were two rainstorms, one behind the other. We'd started running, without rain, then after 100 meters rain started, then after 500 meters it was pouring cats and dogs, then we ran another 400 meters and it was dry again. Bright sunlight with one cloud right above us."

éBack to the Table of Contents

Truck drivers blinded by lights from vehicles behind.
(The Minneapolis Star Tribune, March 04, 2002)

The following item appeared as the weekly Drive Time filler on the front page of the Metro section:

"No spotlight on trucks, please
    When you follow a truck at night, always dim your headlights, the Minnesota Driver's Manual says. Bright light from a vehicle behind will blind the truck driver when they reflect off the truck's large side mirrors."

Prof. J.J. Leigh, who provided this item, commented:

"First, the optical physics is wrong. The size of the mirror does not matter. If it did, then compared to the car driver with smaller mirrors, the truck driver would see brighter images of surroundings during the daytime as well as brighter headlight images at night. Notice that when you view yourself in a mirror, your face doesn't look brighter using a large mirror than with a smaller mirror.

Second, the item seems to imply that it is acceptable to use your "high beam" when following a car. Car drivers are just as likely as truck drivers to be blinded by the bright lights of a following vehicle. Courtesy and safety dictate using "low beam" when following cars as well as trucks.

When Prof. Leigh sent his comments to the Metro page editor, all that came of his effort was the following correction note:

"Cars, too
    Bob Fjerstad and Melissa Daly pointed out that shining bright lights when following any vehicle will blind the driver."

éBack to the Table of Contents