| We are often told that the color of light is determined by its wavelength. So why do violet/purple appear as if they were smooth interpolations between red (high wavelength) and blue (low wavelength)? It's a weird artifact of how the brain reconstructs wavelength from the measurements taken by the retina. The retina has three types of color-sensitive cells (cones). Each one carries a light-absorbing protein that absorbs only certain wavelengths of light. Photopsin I has maximal absorption around 530nm (a sort of yellowish green) and some absorbption towards higher wavelengths, up to 700nm (the red end of the spectrum). Photopsin II has maximal absorption around 530nm as well (so it's green too), but unlike Photopsin I, its absorption drops off rapidly, and does not go toward red. Photopsin III, however, peaks in low wavelengths, around blue. Even though the peak responses of Photopsin I and II are very similar, the brain can use absorption ratios to detect wavelength. If your Photopsin I is absorbing some light, but your Photopsin II isn't, the brain deduces that your retina must be being illuminated by red light. If both Photopsins I and II are absorbing a lot of light, the brain deduces that you're seeing some shade of green (this is why we can tell greens apart better than other colors: we have two receptors for it, but only one for the other colors). If only Photopsin III is absorbing light, but the other two aren't, your brain deduces that your retina is being illuminated by high frequency blue light. Other color perceptions arise as combinations of different wavelengths. If all your Photopsins are absorbing light, the brain deduces that you're seeing a combination of all visible frequencies, white light (this is why prisms can split white sunlight, and why we have rainbows). Similarly, If only Photopsins I and III are absorbing, you must be seeing a combination of red and blue light, which is what you see as purple. And that's why purple appears to interpolate smoothly between red and blue. Okay, but where does violet fit in? Violet isn't purple (a combination of different wavelength light sources), but a pure low wavelength light. Why does _violet_ also appear to interpolate between red and blue? Well, Photopsin I has its absorption maximum around green - but it happens to have a smaller absorption peak in low wavelengths, around 400nm as well. So very low wavelength blue light is absorbed by both Photopsin I AND Photopsin III: and your brain deduces incorrectly that your retina is illuminated by some combination of blue and red. And that's why violet looks similar to purple, and the color wheel appears to wrap around as wavelength decreases. |