This isn’t a duplicate. I read those pages, but those didn’t answer my questions.

Please watch this.

Does visible light consist of red, orange, yellow, green, cyan and blue? No purple? My definition of purple has an RGB of 128, 0, 128:


If there’s purple in visible light, visible light and a rainbow are different since there’s no purple in a rainbow. If there’s purple in visible light, what’s the explanation behind the difference between a rainbow and visible light?

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    $\begingroup$ RGB stands for Red, Green, Blue - each corresponds to a wavelength. TV screens and computer screens just have 1 red, 1 green and 1 blue and all the colors your TV or computer make are made up by how much of each (red, green, blue) there is for that color. (The blue is actually Violet on the rainbow - your video explains that). So Purple is Red 128, Green 0, Blue 128. The rainbow splits the spectrum but the natural world absorbs part of and reflects part of the spectrum, so there's no purple in the rainbow cause blue and red are separated and they need to be together to make purple. $\endgroup$ – userLTK Mar 21 '15 at 13:19
  • $\begingroup$ Are you asking if what we perceive as the color purple corresponds to a single frequency of light? $\endgroup$ – Alfred Centauri Mar 21 '15 at 14:25
  • $\begingroup$ Yes, and what’s the end of the visible spectrum, dark blue or purple? $\endgroup$ – Michael Dy Mar 21 '15 at 14:40
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    $\begingroup$ I guess just the next vid answers your question: youtube.com/watch?v=R5P6O0pDyMU. Purple is a combination of multiple frequencies $\endgroup$ – image Mar 21 '15 at 15:15
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    $\begingroup$ Two things. First, "Please watch this." No. Make your question self-contained. Second the problem with "Wavelength is the way to describe light, not RGB." is that your question really isn't about light. No, it's not. It's about human perception which maps to wavelengths to color in a non-reversable and non-linearly way. There is no fixed wavelength to corresponds to most of our perceived colors. Yes, each wavelength has a color, but each color is not guaranteed a unique wavelength as they can also be elicited by combinations of light wavelengths. $\endgroup$ – dmckee --- ex-moderator kitten Mar 21 '15 at 15:37

As your RGB value indicates, purple is "a mixture of red and blue".

Since a rainbow is created by an effect that separates the incident light into its wavelengths, and red and blue lie on opposite ends of the spectrum, the light coming out of a rainbow-generating droplet or any other prism does not "contain purple", since the wavelengths whose mixture purple is have been separated.

In essence, you will only see the near-spectral colors occuring in a rainbow - those which are approximately given by a single wavelength, or at most a narrow wavelength band:

Visible spectrum on gray background

The "purplish" colors here are more commonly called violet, but there is a history of debate about whether or not this constitutes purples.

Also, there is the fact that colors on the line of purples are very difficult to faithfully represent in any gamut, and even difficult to actually produce in nature.

At the end of the day, the spectrum is made up of wavelengths, and it is totally arbitrary to call them "red", "blue" or "yellow" or something else based on the way our trichromatic vision perceives them. Physics as such has no need for such color names.

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  • $\begingroup$ There’s purple in that pic. The vid shows there’s no purple in a rainbow. $\endgroup$ – Michael Dy Mar 21 '15 at 13:49
  • $\begingroup$ @MichaelDy: Follow the link to spectral colors. That's violet (or indigo), not purple. (Or it may be purple, depending on your actual definition of purple) $\endgroup$ – ACuriousMind Mar 21 '15 at 13:50
  • $\begingroup$ There’s violet in the link to spectral colors. You said that there’s violet in a rainbow. Dark blue is the rainbow’s end, as the vid shows. $\endgroup$ – Michael Dy Mar 21 '15 at 14:03
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    $\begingroup$ @MichaelDy: Read the links. From spectral colors: "Far spectral violet is very dim and rarely seen. The term also extends to purples" Also, as I say at the end, whether you want to call a specific wavelength "dark blue" or "violet" or "purple" or "greligh" is an arbitrary cultural preference, and not physics. $\endgroup$ – ACuriousMind Mar 21 '15 at 14:07
  • $\begingroup$ Is the vid not very good since it doesn’t say that there’s a bit of violet or purple at a rainbow’s end? $\endgroup$ – Michael Dy Mar 21 '15 at 14:12

For one purple forms by combining red & blue, two extremes on the rainbow. Although a rainbow contains all possible wavelengths, it doesn't contain all possible combinations of them, because wavelengths are arranged in order.

Hence, visible light has purple, also there can't be a purple in a rainbow.

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  • $\begingroup$ Visible light is arranged too right? Red has the longest wavelength. Orange follows after red. $\endgroup$ – Michael Dy Mar 21 '15 at 13:27
  • $\begingroup$ Yes! But what do you mean by visible light? If you mean rainbow then it is correct. $\endgroup$ – paneerpatties Mar 22 '15 at 15:58
  • $\begingroup$ Rainbow. I want to know if one end of a rainbow is dark blue. $\endgroup$ – Michael Dy Mar 23 '15 at 12:31

We perceive more colors than those in a rainbow. The reason is that colors in a rainbow are generated by a single frequency of light, but the full gamut of colors we can perceive depend on a combination of three different frequencies (most humans have 3 different frequency sensors, the cones). in addition, there are other factors that complicate matters. Color perception is pretty complicated subject and there are entire books dedicated to it.

enter image description here

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What few people seem to comprehend is that light is not visible. Some electromagnetic radiation is detectable and we refer to this range of detectable electromagnetic radiation as visible light. This is where most of the confusion arrises. Visible is a term used to describe conscious perception of visual representations. Trees, people and words on a page are visible.

When our eyes detect (not see) light, they send electrochemical impulses to our brains visual cortex. Here, our brain interprets these impulses and creates a visual representation of THE OBJECT, from which the light originates. We SEE the OBJECT (technically a visual representation of the object). So objects are visible, not light.

The color (visual sensation) of the object depends on the wavelength/wavelengths of light it emits/reflects, the cone/cones which detect it, the impluse/impulses sent to the visual cortex and finally, how our brain interprets the impulse/impulses. Different impulses/combinations of impulses, result in different visual sensations.

What I'm describing is how indirect realism affects our comprehension of colors. Purple is not in the visible light spectrum, but neither is red, orange, yellow or any other color. Colors are phenomenal sensations. They only exist in our brains.

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  • $\begingroup$ This appears to be a biology-based answer, rather than a physics-based one. $\endgroup$ – Kyle Kanos Dec 23 '17 at 13:52
  • $\begingroup$ Biology is simply the description of physical processes within the body. One can't get a complete understanding of color, without including the biological aspect. COLOR is a visual sensation. You can't leave out vision. $\endgroup$ – Zane Scheepers Dec 23 '17 at 13:59
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    $\begingroup$ Sure, and in the end, it's all just mathematics. But in reality, different fields have different answers/understanding of phenomena. Here, on this site, we expect answers to be based in physics, rather than other fields (that are actually different). $\endgroup$ – Kyle Kanos Dec 23 '17 at 14:00

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