Visible range of the electro-magnetic spectrum?

Question

The visible range of the electro-magnetic spectrum, we all know is the detectable light with all its colors from 380nm to 740nm.

ROY G BIV, Is it just me or does anyone else question why indigo is considered a class of color just like red,green etc...? To me I can hardly see a difference from blue. It seems the color spectrum goes from violet to blue. Indigo looks like blue to me. I see know real difference to merit another name for a hue that isn't that distinctive or even there at all.

Answer 1

When you really look at it, picking names is a matter of personal preference. The spectrum is continuous. You could break it up into any number of units.

It reminds me of musical scales. We have the octave...that's pretty logical, one frequency double another. But how many divisions do you want?
5 - the pentatonic scale
7 - the white notes
12 - counting the black notes.

We could name notes in between the black notes if we want and have more "colours" in the scale.

Back to your question. You could look at the actualy wavelengths and see is there is some average spacing between R and O, O and Y, etc.
But this gets you into one other problem: human perception is not linear. (e.g. a noise that "sounds" twice as loud as the other is not actually twice as intense, hence the decibel scale.)
So you would have to look for constant ratios.
One last point. Another person that answered mention "true blue". Unfortunately, one colour is no "truer" than another!
Interesting question.

Answer 2

There are only really 3 colours. 3 primary colours that is. (red, yellow & blue) All the rest are secondary colours or a mixture of a number of them. you probably already know all this but anyway. I reckon that indigo is considered a colour simply because they couldnt think of another to flesh out the scene abit. Really there should only be 3......

Answer 3

Indigo was added to accommodate the big gap between true violet and true blue.