# Violet light detection

It is my understanding the the human eye is designed to detect visible light frequencies. To do this, the eye has three cones (three rods as well but I don’t know their purpose) which are stimulated by the incoming light according to its frequency. There is a red cone (low frequency), green cone (medium frequency) and blue cone (high frequency). Red light (lowest frequency of visible light) only stimulates the red cone which the brain interprets as RED. Yellow light stimulates the red AND green cones equally which is interpreted as YELLOW. Obviously, blue light stimulates the blue cone only. What about violet? Violet light is the highest frequency of the visible spectrum. One would assume that it is stimulating the red and blue cones (similar to the way yellow does red and green) but this seems illogical. Magenta light is the secondary light color created by mixing red and blue light so if the red and blue cones are stimulated equally, then, the brain should interpret that as MAGENTA. Also, if that were the case, why would higher than blue frequency light (violet) stimulate the low frequency cone (red)? I am sure that my problem lies in my misunderstanding of rods and cones. Please help.

until an expert gets here, i’ll fill in with my guess:

if you look at the response curves for each cone, there is a good deal of overlap. a single frequency violet light would stimulate both cones. i think after that, there’s a bit of post processing the signals coming from each cone to assign a perception of color to the light we detect.

http://www.photo.net/photo/edscott/vis00010.htm - some graphs of the curves

and a post thought -

is the phenomenon that red and blue “mix” to produce violet due to the way our cone response curves overlap? example: red and blue striped disc spinning rapidly to produce the perception of a violet colored disc: obviously the wavelengths of the light emitted from each portion of the disc do not combine to form the violet wavelength. is the disc simply mimicing the response that a pure wavelength violet source would produce (ie a little of red, a lot of blue)? sorry for the hijack

Welcome to the boards, both of you!

meatpropeller your hijack isn’t a hijack, within is the answer to the question.

Violet light isn’t red light and blue light put together. Violet light is higher frequency than blue light. (Look at the first figure of meat’s link and notice the color above 400 nm.) Violet light is absorbed by the blue cones.

You don’t get magenta by mixing blue and red light the same way you do if you mix red and blue paint. The light you’d see from a magenta light bulb is emitted light. The light you’d see from a magenta painting is reflected light. The magenta paint absorbs some wavelengths, reflecting the rest. So for magenta, the material is actually absorbing yellow and green light (I think).

I think you are mixing up the visable light color spectrum with the pigment color spectrum

red and blue make violet in the pigment color spectrum. in visable light, violet is just a frequency above blue.

This thread was accidentally started twice. Both copies had several replies, so I have merged the two copies.

bibliophage
moderator GQ

There are three different pigments in each of the three cones - alpha, beta and gamma rhodopsin.

The absorption spectra of each is a curve, like a bell distribution. The blue pigments absorption spectrum peaks at 440 nm (blue-violet) and tails off in either direction about this point. At say, 400nm (violet) the pigment still absorbs but only at half that of at the peak.

So the eye can detect pure 400nm light, but not as well as blue.

The sight site cited above is broken, here is another (warning: PDF) with the absorption spectra.

You do get magenta light by mixing red and blue light - try playing with colours on your PC screen and you’ll do it fairly easily. Magenta is basically white light minus the green component.

You can come up with what you might call “process violet” this way - otherwise your RGB screen couldn’t show you anything violet. But if you stick this through a prism, it’ll split back into red and blue - whereas if you shine white light through a spectrum and mask off everything but the violet with a filter or even a card with a slit in it, the violet beam that results can’t be split by another prism.

Similarly, you can get yellow light in one of two ways: Mix red light and green light, or get yourself a pure yellow source - a sodium salt on a flame-wire, f’rinstance. If you tweak your red and green lights right, your eye won’t be able to tell the difference, but a prism will split one but not the other.