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Why rainbow indigo?
It is often said there are seven colours of the rainbow but to my way of thinking there are six. Red, yellow and blue are the primary colours; and orange, green and purple the secondary colours where the primaries blend into each other. Why then is indigo given special recognition? It seems to me just an extra 'sub-primary' slotted in between blue and purple. Does anyone know the reason?
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Aren't there an infinite number of "colours" in the rainbow.The visible spectrum is just part of the electromagnetic spectrum that we can see (from red, 750nm to violet, 380nm). This spectrum is continuous and so there must be an infinite number of "colours" which fall into the general classification of red, Orange, Yellow, Green, Blue & Violet.
Interesting answers thank you. I think you are all right, though perhaps in different ways. When speaking of a rainbow one generally thinks in terms of seven colours, the primaries and the 'in between' shades. No doubt there can be an infinite number, as mjd says. I note you emphasise the word LIGHT, nightmare. Perhaps I'm missing a point, but I always thought that red and green (or red, yellow and blue) made brown. It was indigo that really puzzled me and why it deserved a special mention - maybe squarbear has answered that one. Many thanks.
From wikipedia:
Indigo was defined as a spectral color by Sir Isaac Newton when he divided up the optical spectrum, which has a continuum of wavelengths. He specifically named seven colors primarily to match the seven notes of a western major scale, because he believed sound and light were physically similar, but also to link colors with the (known) planets, days of the week, and other lists that had seven items.
Looks like he was superstitious about the number 7.
Indigo was defined as a spectral color by Sir Isaac Newton when he divided up the optical spectrum, which has a continuum of wavelengths. He specifically named seven colors primarily to match the seven notes of a western major scale, because he believed sound and light were physically similar, but also to link colors with the (known) planets, days of the week, and other lists that had seven items.
Looks like he was superstitious about the number 7.
There are many myths about colour. For example the idea that there are specific primary colours that match receptors in the eye.
In fact all three types of cone receptors in the eye have a broad response to a range of wavelengths but peak at different parts of the spectrum. The balance between them is perceived as colour.
Any three colours can be primary provided the third cannot be made from the other two. In reality this does mean that they will be spread across the spectrum but there are no specific primaries.
In fact all three types of cone receptors in the eye have a broad response to a range of wavelengths but peak at different parts of the spectrum. The balance between them is perceived as colour.
Any three colours can be primary provided the third cannot be made from the other two. In reality this does mean that they will be spread across the spectrum but there are no specific primaries.
Coldicote
You seem to be getting confused between mixing PIGMENTS (a subtractive process) and mixing LIGHT (an additive process)
Red and green pigments, when mixed, certainly can make brown.
Red and green lights when mixed will make yellow.
If you want to think of the rainbow simply in terms of primary and secondary colours (instead of as a continuous spectrum) then there will only be FIVE colours.
Red, Green, Blue (primary colours)
Yellow, Cyan (secondary colours)
The remaining secondary is magenta. Magenta cannot appear in the spectrum because it is made from red and blue light and those are at opposite ends of the spectrum and do not therefore overlap.
You seem to be getting confused between mixing PIGMENTS (a subtractive process) and mixing LIGHT (an additive process)
Red and green pigments, when mixed, certainly can make brown.
Red and green lights when mixed will make yellow.
If you want to think of the rainbow simply in terms of primary and secondary colours (instead of as a continuous spectrum) then there will only be FIVE colours.
Red, Green, Blue (primary colours)
Yellow, Cyan (secondary colours)
The remaining secondary is magenta. Magenta cannot appear in the spectrum because it is made from red and blue light and those are at opposite ends of the spectrum and do not therefore overlap.
Quote beso
"Any three colours can be primary provided the third cannot be made from the other two. In reality this does mean that they will be spread across the spectrum but there are no specific primaries."
I'm afraid I disagree with that. The definition of primaries certainly means that the third cannot be made from the other two, but it also means that the remaining colours CAN be made from the three primaries.
The selection of RGB as the primary light colours is strongly tied in with the biology of the eye where we have just three types of colour receptors (cones) that are sensitive to red, green and blue. It is our perception of colour that helps define these terms.
"Any three colours can be primary provided the third cannot be made from the other two. In reality this does mean that they will be spread across the spectrum but there are no specific primaries."
I'm afraid I disagree with that. The definition of primaries certainly means that the third cannot be made from the other two, but it also means that the remaining colours CAN be made from the three primaries.
The selection of RGB as the primary light colours is strongly tied in with the biology of the eye where we have just three types of colour receptors (cones) that are sensitive to red, green and blue. It is our perception of colour that helps define these terms.
As I said before. The idea that the receptors are specifically sensitive to red, green and blue is a myth. They all respond to a broad spectrum but have a peak response at different frequencies.
In fact the long wavelength cones peak response is at 580nm which is perceived as orange.
Where the third colour cannot be made by the first two, combinations of the three will be able to produce any colour.
In fact the long wavelength cones peak response is at 580nm which is perceived as orange.
Where the third colour cannot be made by the first two, combinations of the three will be able to produce any colour.
Typical bandwith response (ie 50% relative to peak) of the three types of cone receptors with their perceived colour when viewed as a single wavelength:
Long: 500nm (green) to 620nm (red)
Medium: 470nm (blue) to 570nm (orange)
Short: 400nm (violet) to 470nm (blue)
It is the relative response of these three types of receptors that is perceived as hue. Two hues that are perceived as exactly the same colour can be made up of different combinations of wavelengths of light.
Consequently red and blue in combination with many forms of yellow can also be used as additive primary colours.
Television and computer displays use red, green and blue phosphors and these have come to be colloquially and incorrectly accepted as The Primary Colours.
Long: 500nm (green) to 620nm (red)
Medium: 470nm (blue) to 570nm (orange)
Short: 400nm (violet) to 470nm (blue)
It is the relative response of these three types of receptors that is perceived as hue. Two hues that are perceived as exactly the same colour can be made up of different combinations of wavelengths of light.
Consequently red and blue in combination with many forms of yellow can also be used as additive primary colours.
Television and computer displays use red, green and blue phosphors and these have come to be colloquially and incorrectly accepted as The Primary Colours.
Now back to the original question.
At any particular point in a rainbow the colours are made of a single wavelength so it is much simpler than the more familiar case of colours made by selective absorption. The bands in the rainbow are areas where the response to colours are too similar to discern appreciable differences.
Violet is perceived at wavelengths below about 430nm where only the short cones are stimulated. They are also at their at their peak response at this point.
Indigo is perceived at wavelengths between approximately 430nm and 460nm. As wavelength increases the response from the short cones is falling while the medium cones increase but the long cones are still not responding.
So yes, indigo does have a unique physiological cause as a combination of the short and medium cones with no response from the long wavelength receptors.
Blue is percieved when the wavelength reaches about 460nm. The long cones begin to respond. As wavelength increases the blue tends increasingly toward green. By 490nm the response of the short cones has fallen below both the medium and long resulting in pure green.
At about 550nm the short cones completely stop responding and the long cones surpass the output of the medium cones resulting in yellow.
From 550nm to 580nm the perception is increasingly toward orange reaching the peak response of the long cones at 580nm.
The organge tends increasingly toward red until 600nm where only the long cones respond and we perceive red.
At any particular point in a rainbow the colours are made of a single wavelength so it is much simpler than the more familiar case of colours made by selective absorption. The bands in the rainbow are areas where the response to colours are too similar to discern appreciable differences.
Violet is perceived at wavelengths below about 430nm where only the short cones are stimulated. They are also at their at their peak response at this point.
Indigo is perceived at wavelengths between approximately 430nm and 460nm. As wavelength increases the response from the short cones is falling while the medium cones increase but the long cones are still not responding.
So yes, indigo does have a unique physiological cause as a combination of the short and medium cones with no response from the long wavelength receptors.
Blue is percieved when the wavelength reaches about 460nm. The long cones begin to respond. As wavelength increases the blue tends increasingly toward green. By 490nm the response of the short cones has fallen below both the medium and long resulting in pure green.
At about 550nm the short cones completely stop responding and the long cones surpass the output of the medium cones resulting in yellow.
From 550nm to 580nm the perception is increasingly toward orange reaching the peak response of the long cones at 580nm.
The organge tends increasingly toward red until 600nm where only the long cones respond and we perceive red.
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