This comes up everytime I teach physics for elementary education majors. The curriculum I use (Physics for Everyday Thinking – which is awesome) says that the colors in white light are ROYGBV (Red-Orange-Yellow-Green-Blue-Violet). Typically, I will get a student that says “Hey! What about indigo? Shouldn’t it be ROYGBIV?” My first reaction to this was “huh?” Really, does it matter? Here is the spectrum you would see looking at a white light source.
You could break this into as many or as few colors as you like. So, it doesn’t really matter. But this leads to a great question: Who did this, and why is it that way (I guess that is two questions). Let me add a third: why do all the textbooks have the ‘I’ for indigo (although they all don’t, but many of them do).
When this first came up, I had no idea why it was this way. But, I am an expert googler – so I found the answer (at least an answer I liked well enough to share with the students).
Basically, Newton wanted there to be 7 colors because 7 is such an awesomely cool number. Mainly, there were 7 objects you could see in the sky. Ask your students if they can name these 7 objects, its fun.
My students were also surprised to see the connection between the days of the week and the seven planets. It is easier to see this connection in a language other than English. First, the objects. In Spanish, they are (here are the planets in tons of languages):
Sol, Mercurio, Venus, Luna, Marte, Jupiter, Saturno. And the days of the week are: lunes, martes, miercoles, jueves, viernes, sabado, domingo. Ok, all these aren’t obvious in Spanish, but if you put this together with English you can see the connection.
I didn’t answer one question. Why do so many of the textbooks list the ‘I’? I think this is a problem of the way textbooks evolve. They kind of start with a seed and slowly change. They carry over stuff from previous versions (maybe even other titles) for sometimes unknown reasons. You can see this a lot in the intro physics texts. Why are they the way they are? Do you have to do kinematics before forces? The traditional way is to have it that way. Of course, there are some textbooks that don’t follow this pattern.