Gootar has a cool little tutorial that compares the coincidental similiarities between the visible light spectrum and the tones and pitches of music: Anyone delving into music theory knows there are seven notes in a musical scale.

Using a C major scale and an E major scale as examples we have the patterns noted above. The center section… Root, second, third, fourth, fifth, sixth and seventh are the scale degrees relative to the root note (the root note can be any note). Chords are a combination of three or more notes in a scale. Major chords are comprised of the Root, third and fifth notes of any major scale. Major chords are the basis of all other chord structures.

There are also an abundance of other frequencies (tones/vibrations) in the sound spectrum, but the notes in the scale will repeat themselves the further higher and lower you go up and down in frequency. Notes of the same tone but different pitch are called octaves, for instance, “A-440″ is an abbreviation for the official government standard of musical pitch in the United States. Related to pianos, it means that the strings for the “A” just above “middle C” should vibrate at 440 cycles per second. Higher and lower A’s will vibrate at 880 and 220 respectively. (notice the frequencies are equal divisors of each other… either double or half)

The light waves (electromagnectic spectrum) are also doing this, we just can’t see the higher and lower frequencies because unlike intermittent sounds, we are constantly being bombarded with light, radio and other waves. If we could actually see all of them, it would probably appear as just a big soupy mess… an analogy using sound would be like hitting all of the piano keys at the same time. I guess our brains just picked out a nice section of the electromagnetic field (the scale of light) and made it visible. So, there are seven color frequencies and seven sound frequencies used by our eyes and ears for sight, hearing, art, music, painting, song writing, etc.

Then, Gootar points out the mystery underlying color and sound:

The first, third and fifth colors of the rainbow are Primary colors… red, yellow and blue.
The first, third and fifth notes of a major scale are Major chords… root, third and fifth.

Something quite amazing happens when we tie all of this information together (that’s Physics, Optics, Light, Sight, Sound, Hearing, Art, Music, Neuroscience) and line up the two groups of seven up together…

Primary Colors are Major Chords!