Difference between ‘violet’ and ‘purple’

For example, when you see pure red light, only the “red” cones are activated, which tells your brain to create the impression of red. On the other hand, when you see pure green light, both “green” and “red” cones are activated, but your brain is smart and knows that “a lot of green activation and a bit less red activation” is in fact just pure green colour, which is what you see.

The “red” cones have an interesting additional property. They have a small bump of absorption around the short-wavelength (violet) end of the visible spectrum. When violet light hits your retina, both the “blue” cones and much less the “red” cones are activated, but your brain is smart and knows that “a lot of blue activation and a bit of red activation” represents violet.

Purple is not a spectral colour

Not all colours we can see are in the spectrum displayed above, the most notorious example being brown—there is no such thing as a brown wavelength. When you see an object, typically a mixture of different wavelengths reaches your retina, which causes the cones to be activated at a ratio not achievable by a spectral colour.

Our brains are very efficient in how they interpret this mixture, and, as a result, we are able to see several million different colours, most of which can only be formed as a mixture of different wavelengths and are not present in the spectrum.

As we noted at the beginning of the article, purple looks more “reddish” than violet, and that’s absolutely correct; purple is formed by mixing red and blue at a ratio close to 1:1, whereas violet is perceived by your eyes as containing more blue than red. However, as you can see from the picture above, no spectral colour stimulates the “blue” cones and the “red” cones at the ratio of 1:1 without also stimulating the “green” cones.

In other words, violet is a “real” colour and purple is not; you can have a source of monochromatic violet light, but everything that looks purple must emit both red and blue light.

Purple and violet look similar only to humans

If you take a look at the distance between violet and blue in the picture of the spectrum above, it is about the same as the distance between green and orange. Purple is a mixture of red (which is at the opposite side of the spectrum) and blue (which is relatively far from violet), so it is, technically, a completely different colour.

The reason why purple and violet look similar to us is the particular absorption curve of our “red” cones, which most other animals don’t share. This means that for other animals, purple and violet may look completely different!

Now imagine a violet flower petal with a purple pattern on it. Depending on the particular shades, this pattern might be completely invisible to us, while many other animals could see it as clearly as we can see an orange pattern on green background. Even cameras wouldn’t help us; they capture the same red-green-blue information as our eyes do, so even taking a photo of the petal and editing it in Photoshop would not uncover the pattern. Quite fascinating, isn’t it?