[Physics] Why don’t we percieve chords like we perceive the mix of two light waves?

acousticsopticswaves

Why is the combination of two light waves (red, yellow) percieved as the same color as the arithmetic mean of their frequencies (orange) while we percieve two musical notes at the same time as just those two waves stacked on top, and not the mean of those frequencies?

Best Answer

Because 16,000 is greater than 3.

We only have 3 sorts of detector (called cones) in the eye, sensitive broadly to red, green and blue light. So a mix of red and green light excites the red and green cones. But yellow light, in between red and green, also excites the red and green cones, and the brain can't tell the difference. So it's not the mean of the frequencies - a red-blue mix is different from green - but there is some averaging going on.

But the ear has 16,000 hair cells each sensitive to a particular frequency, so the brain gets a whole lot more information. A 256 Hz C will excite the 256 Hz hair cell, but a mix of 242 Hz B and 271 Hz C# will excite those two receptors and not the one in between.