If orange has a frequency in the visible spectrum that is 620nm, how does light that is 2/3 red at 750nm and 1/3 green at 570nm also make orange?
In: Physics
You have 3 types of color sensitive cells in your eye (this is where the red, green and blue primary colors come from). As you already understand light frequency [here’s a diagram to show the frequencies they are sensitive to](https://qph.cf2.quoracdn.net/main-qimg-870ea705b8722d16375ec0f546ea0a37-lq). As you can see there is a lot of overlap.
If you look at where 620nm is it’s triggering the red cone cells twice as much as the green. So if you have a mix of 2 different light frequencies that trigger the red and green cone cells the same as that it will look the same color.
Your eye is full of photoreceptive cells called rods and cones. The rods help you see shapes in low light, while the cones have different receptors to sense color. The cones have three distinct types for sensing red, yellow, and blue light. All your brain can detect is the ratio of these signals coming from each spot in your field of vision, similar to how the pixels on a TV create a color by combining a number between 0 and 255 for each primary color (with 0, 0, 0 being black and 255, 255, 255 being white.)
There’s no receptor for orange. Your brain sees orange when there is roughly equal light absorption by the red and yellow cones and far less by the blue cones. This can be accomplished because the photoreceptors aren’t sensitive to exactly one wavelength, but have a function of sensitivity, with red cones absorbing extremely well at 600nm, but not as effectively at 500 or 700nm. The absorption profiles of the cone cells overlap, with orange light at 575nm triggering both the yellow and red cones. This would create the same effect in the brain as sending pure light of yellow and red in concert (in practice there’s no one wavelength that triggers one type but not the other.)
This chart should help envision what I’m trying to describe:
http://hyperphysics.phy-astr.gsu.edu/hbase/vision/imgvis/colcon.png
See [this diagram](https://en.wikipedia.org/wiki/File:CIE-1931_diagram_in_LAB_space.svg) which allows you to work out how colours mix additively. Draw a line from 750nm to 570nm and you’ll see the point dividing the line in a 2:1 ratio is in the reddish orange region.
Note that 750nm is right at the limit of human vision so we see it only very dimly. If you meant 2/3 red measuring in watts then it will appear much dimmer than 1/3 green. If you measure in lumens, which means weighted to account for human perception, the result would be different.
Your eyes have red, green, and blue cone cells. The cone cells detect light in partially overlapping ranges. Orange is when the red cone cell is being activated slightly more than the green cone cell. Yellow would be red and green being activated about equal levels.
[Diagram](https://i.stack.imgur.com/SUUDl.gif)
You can make what appear to be any color with just varying strength of red, green, and blue light. This is what a screen does, but it is different from the actual wavelength of light found in nature. Your brain just interprets it the same way.
If you have an object that only reflects 620nm light, it will loon orange under normal circumstances, but if you place it in a room lit only by 750nm, 570nm light, and 450nm, the object will look black despite the room seemingly being filled with white light.
Those are wavelengths, not frequencies. Visible light has a frequency range of about 400-700 terahertz
Because your mind fabricates colors. 🙂
Not all colors have a wavelength: Pink, Brown, … (These are calles extraspectral colors)
This same principle is important for lightning:
You could create white light by combining Red, Green en Blue LED lights, however, that white light would still only contain the wavelengths of only Red, Green, and Blue, while real white light contains the entire visible spectrum. (See also CRI value of a light, Color Rendering Index.)
Because your eyes only have three types of colour sensors – red, green and blue cones. The colours you see are about how much those sensors are each stimulated. And their ranges overlap, so a single frequency can stimulate more than one.
Orange light stimulates both the red and green ones, in a certain mix, and your brain calls that combination of signals “orange”. BUT. If you see light that’s actually a mix of red and green frequencies, and the two stimulate the cones to the same relative amounts as orange light does, you also see orange. Your eye and brain have no way of telling whether you’re seeing one orange wavelength or a mix of two others – just what the overall balance of stimulation is.
We have cells in our eyes that react to different frequencies of light, red, green, and blue. The thing is they are not perfect so they not only react to 750nm or 570nm, it’s more a range and it overlaps, orange light excites both the red and green cells a bit at the same time, so if you use pure red and pure green you can excite those cells at the same level that orange light does.
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