It is said that people and digital cameras have color sense only for red, green and blue. So when we see violet/purple in a rainbow, whether with our eyes or in a photo, we should be seeing a color mixture dominated by red and blue, with less green.

Is this correct and if so, where is the red coming from?

And do rainbows have violet for the same reason black lights are violet?

Note: yes, there are similar questions such as this one but I don't think anyone has asked this question directly.


At first glance, one might look at Wikipedia and think that a naive understanding of "humans have perceptors for red, green and blue" is correct:

However, different sources look different.

Another source that extends the "red" response toward UV below 420nm shows a "bump" of higher red response (note: there are four curves because the second one represents rod perception, which gives us limited night vision):

A third source confusingly shows both red and green bumps and cuts off the response on the left around 40%:

Anyway, this "red bump" would explain why we see purple in a rainbow instead of blue: violet activates red cone cells in our eyes, if only a little bit. This also explains why black lights look purple, as their peak frequency is usually below 430nm.

What about digital cameras? It turns out that they often have some kind of "red bump" too so that violet light below 430nm will activate the camera's red sensor.


The human eye exhibits a property called metamerism, which means that although it has sensors only for red, green and blue, it produces a signal for yellow for example when simultaneously illuminated by green and red light. Similarly, red and blue together yield magenta and by trimming one intensity or another, you can get purple out of the combination.

Because of this property, the human eye can be fooled into perceiving the full spectrum of colors in nature with only an R/G/B source.


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