Sir Isaac and His Seven Colors

Lately we've been rockin' out in the Riddy household to They Might Be Giants' "Roy G Biv", and it's got me thinking about Isaac Newton and the rainbows we have inherited from him. 

I say that we have inherited the rainbow from him because our actual viewing of real rainbows is filtered based on our expectations.  We "know" there are seven bands and we have seen them many times in various works of art.  Why seven colors though?  The rainbow is a continuous spectrum from the lowest frequency the eye can detect (red) the highest (violet) so nature has not divided the colors into bands. 

Human eyes, however, can only detect three different frequency bands based on the three cones in our eyes (commonly called Red, Green, Blue, although this is an oversimplification).  All colors are an interpolation of those three signals (remember we are talking physiology here not physics).  Take a good look at this close-up of a rainbow:

I can only make out five uneven bands without sharp borders.  It turns out Newton originally only identified 5 colors: red, yellow, green, blue and violet.  Maybe we see five because we get one for each of our cones plus one each for when two neighboring (frequency-wise) cones are being equally stimulated?  (All three cones equally weighted give us the "color" white, of course). 

So where did Roy G Biv come from?  Newton wanted to make the color spectrum fit with the music spectrum of seven notes (click here for Newon's color wheel with notes attached)!  Ever since, artists dutifully draw their rainbows with seven colors and we convince ourselves that we see seven (since it's a continuous spectrum I imagine you can train yourself to identify many arbitrary number of bands as you want).

Beware the difference between what you think you see and what you actually observe!