Re: Why are kids taught that a rainbow has 7 colours?

Brian,

It is good to hear from someone "down under". My son spent time in Adelaide and loved so much of your country. I was not fortunate enough to visit him at that time. Since your question has many facets, I will answer it in pieces. I hope this will be a help to you.

We know very little concerning Newton's private life ... much less about any of his lady friends or their wardrobes. Today, of course, those things would be on cable television for all to discuss. We also know little about the actual thoughts of individuals, but more about an age-old quest to discover something new while not departing so far from the norm as to have one's discovery disregarded. Acceptance by his peers was not Newton's strong suit, but it must have been a factor in his work.

I will ignore your reference to the "art colors" since these are reflected colors and do not represent the colors of the rainbow or the colors seen using a prism. Pigments and filters produce color by absorbing portions of the light source and allowing other portions to either reflect or transmit to our eyes. Adding pigments or mixing crayons will produce a darker look. By contrast, adding or mixing colors of light will always produce a brighter look. While individuals will differ slightly (and some will have defects), the perception of color depends on the response of the red, green, and blue cones in our retina. These correspond to the red, green, and blue (RGB) light producing phosphors in a TV screen. This makes red, green and blue the primary colors of light, but not of paint. All three saturated colors are considered dark. If three cans of R, G, & B paint were mixed, the result would be a very dark mud. However, the mixing of R, G, & B light becomes lighter and with the correct proportions would be considered white. For a complete discussion of this and the human perception, consider Light and Color by Overheim and Wagner, written in 1982.

We do know that, during Newton's time, there was a strong belief that the physical world was tied together very neatly and followed a fixed pattern. That much has not changed so much. When Newton discovered that white light could be separated into colors (and merged again into white) he most likely wanted the number of colors to follow the accepted seven notes of the harmonic music scale. It is recorded that Newton sensed a connection between sight (light) and sound (music). But why of all the notes would there be seven before repeating? Why seven days of the week ... seven planets (known at that time) around the Sun ... seven as the perfect or complete number from biblical reference? Today we still have the seven wonders of the world, both modern and ancient. Recently we have inflated this concept to have "Top Ten" lists and well as "101 Places to Go Before You Die".

Why indigo? Well, indigo was a dye known to the ancients; so it was not a strange or new color. Indigo was important due to the few sources of any blue colors. It was extracted from both plants and animals (shells) until manufactured synthetically about 100 years ago. As any wearer of denim knows, it does not adhere to cloth permanently. And to justify his naming, Newton may have been one of a minority of individuals that can actually distinguish three hues of the spectrum on either side of green. I do not. My personal observation, from 38 years in the classroom, is that whether 7,6 or fewer colors seen, individuals will disagree where the separations actually occur. For more on indigo, see http://en.wikipedia.org/wiki/Indigo_dye.

In closing, you may be relieved to know that many people agree with you on the existance of indigo in the spectrum. There are text-references to the six colors of the rainbow and refer to ROY G BV. Many perceptions are continued for generations following a better way of understanding. Maybe indigo is the most common form of color blindness, and there will be a few who wonder what that seventh color is. I personally wonder if it is indigo or violet that I cannot see.

Gene Ewald