With prime colors blue and red makes violet, but when light is split with a prism it makes secondary colors between prime ones, except violet is on the very end and not in contact with red light, only blue. Why is this?



With prime colors blue and red makes violet, but when light is split with a prism it makes secondary colors between prime ones, except violet is on the very end and not in contact with red light, only blue. Why is this?

In: Physics

Hi 🙂

Our eyes have three types of color receptors for red, green and blue. (And some for just brightness.) Kind of like printers make up all the colors mixing ink / combining ink dots close next to each other.

But it’s not as simple as that. The receptors for red are also a bit sensitive in the other range.

When the receptors for red and blue detect something, but not the receptors for green, our brain perceives violet.

First of all there is – somewhat – of a misconception in your question. As you named Blue and Red to be primary colors you’re probably talking about additive coloring. That is when one uses light. But Blue and Red doesn’t ‘make’ Violet, it ‘makes’ magenta. (subtractive would yield purple)

Both magenta and purple are entirely made up by the brain, they are no spectral colors.

However as you mentioned yourself violet is actually part of the visible spectrum of electromagnetic waves. Thus it is a color that corresponds to a particular wavelength. Also called a spectral color. Violet is at the blue end of the visible spectrum, thereby there is no red in it (as you yourself pointed out).

So technically they are all different colors, magenta and purple are “fake” and violet is “real”, in the sense of electromagnetic waves.

However: The human eye has 3 different color-receptors that each correspond to a certain range in the blue, green and red area of the visible spectrum. The blue cones do encompass violet as well, not just pure blue. In fact the blue receptors peak at around 450nm which is the boundary between violet and blue.

Also in the case of violet, the red receptors actually do fire as well, making the brain interpret some form of red even though there is none in violet itself. So given the right mixing it should indeed be possible to trick the brain into thinking something is violet when it’s actually a real mix of blue.

TL;DR: Blue and Red doesnt “make” violet, it at best approximates something the brain interprets as violet, so it’s just as “unreal” as purple or magenta. The real violet however is a spectral color without red in it but the L-receptor (red) of the human eye fires anyway. So one is a “trick” and one is the real deal.

Light comes in a spectrum. Rather than having distinct colours, it comes in [a whole range of colours](https://en.wikipedia.org/wiki/File:Linear_visible_spectrum.svg) based on the wavelength of the light involved. Depending on the person and circumstances adult humans can generally see light of wavelengths 400nm to 700nm.

But detecting the precise wavelength of any bit of light hitting something can be a bit tricky. Instead, our eyes cheat. The cones in our eyes (the cells which detect colour) generally come in three types; S-cones, M-cones and L-cones covering short-wavelength, medium-wavelength and longer-wavelength light respectively.

Each cell responds to a broad range of light, but with different intensities, given by [something like this](https://en.wikipedia.org/wiki/File:Cone-fundamentals-with-srgb-spectrum.svg). So 450nm light will get a big response from the S-cones, a small response from the M-cones and a slightly smaller one from the L-cones. 600nm light will get a big response from the L-cones, a decent response from the M-cones, and nothing from the S-cones.

Those signals then go to our brain, which takes the responses from each of the three types of cone, and uses that to figure out exactly what wavelength of light hit them.

So if it gets a big S-cone signal, and small M-cone and L-cone signals, it says “aha, this must be violet.” If it receives large M- and L-cone signals and no S-cone signal, it works out the light must be green.

And so with only 3 types of light sensor our brain can pick out any colour – which is pretty clever. Importantly, the brain is never told which wavelengths of light the cones received; only how big the response signal was; so for example a half-strength response from the S-cones could mean ~420nm light or ~470nm light, the brain needs to check the signals from the other cones to know for sure.

But this means we can *trick* our brain, which is how monitors, screens and other multi-colour displays tend to work. What we do is get three coloured light sources, but put them so close together our brain can’t tell them apart. So when the light from each source hits our eyes, each of the cones give an appropriate response from the three different wavelengths of light, but the brain combines them into a single colour (because it doesn’t realise it is being tricked). If you look close enough at a screen (easiest with a really big one designed to be viewed from far away) you can eventually get close enough that your eyes can distinguish the three lights, and you see them individually. As you back away they blur into one colour.

So now let’s talk about violet.

We’re looking at around 440nm wavelength. Going [back to our diagram](https://en.wikipedia.org/wiki/File:Cone-fundamentals-with-srgb-spectrum.svg), our eyes will pick up violet by sending a strong S-cone signal and weak M- and L-cone signals, and our brain will combine that back into violet. But we can also get that by combining a strong blue light and a weaker red light. The strong blue light gives us our S- signal, but it gives us too much M- and not enough L- for violet. The red light then adds a bunch to the L- signal (and a bit to the M- signal), so now our brain receives “strong S-,” “a bit more L- than M-” and says “aha, this must be violet!”

So red and blue combine to make violet, but only when tricking the brain.

And our brain does a weird thing where it tries to connect up the two ends of the colour spectrum (remember there is violet at one end and red at the other), into a [colour wheel](https://en.wikipedia.org/wiki/File:Hsv_color_circle.svg). And to do so it needs to create a colour that doesn’t have a corresponding wavelength of light – a colour between red and violet, a mauve colour.

a violet from the screen and a violet in physics (say a violet laser) are different. one is a mixture of red and blue wavelengths, one is a single wavelength at violet.

You see them as same colors, because our eyes are bad. Eyes only see three colors, but they have overlapping color regions. it is the relative weight of the eye receptors that will make your color impression. The single wavelength violet will activate the red cells and the blue cells in a specific ratio. The screen light aims to imidates this ratio by providing red and blue light such that your eye gets activated by the same ratio.