How does additive color mixing (RGB) on a monitor or LED light etc. simulate different wave lengths (frequencies) of light if it is just mixing different amplitudes of three discrete wave lengths?

Question

Having a background in sound I am probably coming at this all wrong but if you mix a 1kHz sine wave with a 2kHz sine wave at various amplitudes you will get various different sounding composite sounds but at no point will you be able to emulate say, a 1300Hz sound. How is it that mixing Red Light at 462 terahertz (or whatever) with green light at 545 terahertz (these are numbers I am just pulling off google) at the same amplitude can result in a perceived frequency equivalent to 516 terahertz or as we know it ‘yellow’?

Is it that the ‘yellow’ we experience from Additive colour mixing is not the ‘true’ yellow we see in the rainbow? Is it our eyes that make up the colour based on the input of two discrete light sources interfering with each other?

In: 10

Answer 1

Our ears have hundreds of tiny frequency-sensitive hairs with attached nerves in the cochlea, so we can very sensitively hear the difference between sound frequencies just fractions of an octave apart.

By comparison, our eyes are pretty much tone deaf. If our eyes were ears, we’d be able to hear just three very broad pitches: “low” (red), “medium” (green) and “high” (blue)

These three pitches would get mixed into different “chords” we call colours: purple is red + blue, yellow is red + green, orange is red + less green, white/grey is red+green+blue, and so on.

But since our eyes are so tone deaf, we can’t see the difference between, say,

* two tones, one “low” and one “medium” (red light + green light)
* a single “medium low” tone (single frequency yellow, eg from a sodium lamp).

These combinations look pretty much the same to us. A spectroscope could tell the difference – eg, a prism could separate red+green into a red band and a green band, but the identical-looking “pure” yellow would yield a single yellow band.

Whether red+green is “true yellow” depends on whether you’re talking about colours as things humans perceive (eg, if you’re choosing colours for a design or art piece), or about (say) “colours” as bands on a spectroscope (eg, if you’re doing analytical chemistry), but it sure looks yellow to us, and that’s often good enough.