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
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.
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