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Why is rainbow VIBGYOR?

  1. Jan 16, 2016 #1
    I got to know 'bout primary colors
    Even after knowing that red, green and blue can bring infinite color combination; why do we still 'violet', 'indigo', 'yellow' and 'orange' as a part of the colors of rainbow while there are also different other colors between any two of them which are not considered as a part of 'the colors of rainbow'
  2. jcsd
  3. Jan 16, 2016 #2


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    Staff: Mentor

    I suppose it's because we have simple, commonly-recognized names for those colors. It's not meant to exclude the many intermediate color shades in between. The simple color names don't have precise definitions anyway. They already include ranges of shades.

    By the way, I don't know where you're from, but English-speakers probably refer more commonly to Roy G. Biv than to VIBGYOR ?:).

    Aha, now I see there's a chain of high schools in India with that name. Also a band named Vib Gyor in Leeds, England. If you're connected with either of those, carry on... :biggrin:
    Last edited: Jan 16, 2016
  4. Jan 16, 2016 #3
    Vibgyor seems to be older than roygbiv, its first written occurrence is 1743. [1]
  5. Jan 16, 2016 #4


    Staff: Mentor

    I think that is a very difficult question having to do with our eyes and brains, not physics.

    I wager that if you ask 10 people how many colors they see in a rainbow, that their answers are similar.

    I also expect that other animals and insects that have different eyes and visual spectra, see different colors and different number of colors than we do.
  6. Jan 16, 2016 #5


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    Staff: Mentor

    We recently had a very long discussion about color vision: https://www.physicsforums.com/threads/what-decides-the-colour-of-light.842780/

    Decomposing the rainbow in seven colors is completely arbitrary, and is probably due to belief in numerology.
  7. Jan 17, 2016 #6


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    Gold Member

    For clarification, Red, Green and Blue, when mixed in various proportions, can cause the brain to perceive any colour including white. It is a psycho-perceptual effect. But the mixture does not physically produce that colour.
    So the colours of the rainbow, each being a different wavelength monochromatic light, can be perceived by mixing the three primaries in various proportions. The formal way of depicting this is using Maxwell's Colour Triangle. As a a matter of interest, most colours can also be perceived by using two "primaries" located opposite each other on the colour triangle.
  8. Jan 17, 2016 #7


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    Primary colours and the wavelength a monochromatic light are diametrically opposite concepts, yet they are often considered to the the same thing. Wrong. Wavelength just refers to the wavelength of a particular part of the EM spectrum. A fairly simple instrument can measure objectively the wavelength spectrum of any light source (a prism and a photodetector). The word 'colour' instantly takes us into the psychology of vision and the way the human eye analyses the light it sees; it's subjective. We seldon see monochromatic light and most colours we see will have a complicated spectrum of many wavelengths. It so happens that the eye uses three different receptors and each one has a different sensitivity to the optical wavelengths (although they all three have some sensitivity over more or less the whole of the visible spectrum. Our sensation of colour is based on the relative amounts that each of the three sensors respond to any particular input spectrum. It's the relative weighting of these sensor outputs that we use to classify the 'colour' we recognise. Years before the proper theory of colour vision, it was found that mixing pigments could produce a range of colours by subtractive mixing. Then Newton and others found that you could also mix different wavelengths of light to produce colours. Using three (so called) primary light sources, you can match most colours (TV is all about matching well enough for viewers to accept what they see as reasonable). For practical reasons (maximum achievable brightness), the phosphors in a TV display have the colours Red, Green and Blue but do not actually consist of 'pure' spectral lines. Most colours can be displayed / matched with a whole variety of mixes of arbitrary primaries and, as colour display technology improves, a bigger gamut of colours can be displayed.
    PS We quantise the colours we see, when describing them and recognising them and we also quantise the musical notes we use but there is an continuum of wavelengths and frequencies in each case.
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