When two colors get mixed, why does our brain suddenly see it as a completely different color?

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So even if you use sand and not a liquid, and mash yellow and blue together, suddenly you start seeing green. Now, if you were to examine it closely, there would not suddenly be actual ‘green’ specks of sand. It’s still just yellow and blue specks.

So how come we don’t see just see yellow and blue specks? Why does it suddenly turn green?

Same thing with a liquid – why don’t we see splodges of yellow and blue swirled tightly, instead of the blue and yellow turning green? If I examined the liquid closely, would it have actually turned green? Or would it still be separate blue and yellow specks of dye?

In: Physics

2 Answers

Anonymous 0 Comments

You are getting tripped up because you are conflating two different concepts: colour and the frequency of light.

Light has whatever frequency it has, and you are right that after mixing things that reflect yellow and blue light together, those elements continue to reflect yellow and blue light. The light is what it is.

But that’s not what colour is. Colour is your brain’s interpretation of signals from detector cells in your eye called cones. You have three types, one sensitive to blue, one sensitive to red, and one sensitive to yellow frequencies, and that’s it. When you shine blue light in your eye, the blue-sensitive cones fire signals to your brain that say “hey I’m detecting something” and your brain goes “these guys are only sensitive to blue, so we must be seeing a lot of blue”.

But there’s a catch in how these detectors work. They aren’t sensitive to a SINGLE colour, they are broadly sensitive to colours around those colours, and their sensitivity overlaps. So for example, green light stimulates both the yellow and the blue detectors a little bit. When you shine green light your eyes, BOTH the yellow and blue receptors go “hey, I’m picking up something here”, and your brain goes “ok, if light is stimulating BOTH yellow and blue receptors, we must be seeing green”.

But what happens if you shine both blue, and yellow light, simultaneously? Again, both of the detectors fire, and your brain can’t distinguish this from the previous scenario, so you interpret it as green.

This can lead to some whacky stuff, if you think about it. For example, white is definitely a colour, but it doesn’t correspond to any specific frequency of light. White is what you get when ALL detectors fire, which is why you get it by mixing every primary colour.

Black corresponds to no visual stimuli at all, but it’s definitely a colour that your brain knows how to recognize.

And, most strangely…. violet. Violet is the colour you see when both blue and red detectors fire. But red and blue detectors are on OPPOSITE ends of the visible spectrum. So how can a single frequency of light stimulate both red and blue cones? It’s because the blue-sensitive cones have a long tail towards the red, which overlaps with the red.

Hope that answers your question.

Anonymous 0 Comments

What you’re calling “color” is actually light frequency, and our eyes do not see it. At least, not directly. Your eyes can detect three things: “red”, “green”, and “blue”.

You can only see frequencies that are “yellow” because they are similar enough to red and green that they get detected by your red and green sensors. When a “yellow” frequency comes in, it looks like a little bit of green and a little bit of red. That’s what your eyes see.

So if you had a little bit of a red frequency and a little bit of a green frequency, it would look exactly the same to your eyes as a yellow frequency, even though it’s physically a different thing. Your eyes can’t tell the difference.