# Blue + Yellow = Green

## Main Question or Discussion Point

We perceive blue light mixed with yellow light to be equivalent to green light. Does this have more to due with the way the wavelengths interact or some other physical phenomena or is it a biological phenomena

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tiny-tim
Homework Helper
hi mrspeedybob!
We perceive blue light mixed with yellow light to be equivalent to green light. Does this have more to due with the way the wavelengths interact or some other physical phenomena or is it a biological phenomena
(no, we perceive it as cyan (turquoise) light! EDIT: oops! i mean white light! )

there's no interaction between the wavelengths, so the colour an eye perceives may be different for different eyes (for the same light), particularly from different species …

two lights which look different to one eye may look the same to another eye

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We perceive blue light mixed with yellow light to be equivalent to green light. Does this have more to due with the way the wavelengths interact or some other physical phenomena or is it a biological phenomena
It is the subjective perception of your brain in response to which combinations of different colour sensors within the eye that react to the different coloured components of the light.

Very simplified, it is like this: You have three types of sensors ("cone cells") that are sensitive to three different wavelengths of light. If sensor type 1 reacts to incoming light, the subjective experience is defined as "red". If sensor type 2 reacts, you experience a colour which we define as "green". And then similar for blue.

If your sensors for red and green are both activated at the same time, the subjective colour perception you experience is called "yellow". The brain doesn't primarily percieve it as "red + green at the same time", but instead something different, i.e. "yellow".

It has nothing to do with interactions between different frequency of light.

D H
Staff Emeritus
First a comment about the title of this thread. Your blue+yellow=green comes from a rather outmoded color model in which red, yellow, and blue are the primary colors. Your TV and your computer screen use red, green, and blue as the primary colors (additive color) while printers use cyan, magenta, and yellow as the primary colors (subtractive color).

Wavelengths don't interact. Color is mostly an illusion. There are very few pure colors (light with a unimodal, sharply peaked spectrum) in nature. Instead, the light coming off objects are multimodal with rather broad peaks.

Suppose you see someone on TV wearing a bright yellow outfit. There is *no* yellow coming from your TV. It is just a mixture of red, green, and blue light that your eye interprets as yellow. You have three different kinds of cones in your eyes. Each kind of sensor is sensitive to a range of colors, with different kinds of cones having different response curves. It is the combined response of these sensors that you interpret as "color".

rcgldr
Homework Helper
Blue + Yellow light would be percieved as white, bluish, or yellowish, depending on the ratios (additive color). Blue + Yellow paint would be perceived as some shade of green (subtractive color).

Thank you all.