Explain purple the last colour of rainbow
Answers
Answer:
It is true that Violet is a ‘spectral colour’ - meaning that it has its own wavelength (about 380 - 450 nm) - while Purple is not. Purple is what we perceive when blue (450 - 495 nm) and red (620 - 750 nm) are mixed. So in a sense, purple is not ‘in the rainbow’ - we just see it there!
Most people (particularly men) are either unable, or unwilling, to tell the difference between violet, indigo, purple, lilac, lavender, mauve, magenta, fuchsia etc. (A joke when I was growing-up was that you could tell a guy was gay if he could name more than six colours! Either that isn’t true - or I just outed myself spectacularly) So when someone says “red, orange, yellow, green, blue, purple” - they are actually misidentifying “indigo/violet”. It is just a nomenclature/interior-design error. (and nobody much cares!)
So why do we ‘see’ purple when it isn’t a spectral colour if it needs photons from opposite ends of the visible spectrum to cooperate to make it?
This is how the cones (colour receptors in our eyes respond to light of varying wavelengths.) As you can see, the S(hort wavelength)-Cones have a wide but well-defined response at the blue end of the spectrum. Likewise the M(edium wavelength)-Cones respond to widely-spread range on the yellow-green. But the L(ong wavelength)-Cones have that odd, bimodal response you can see as the red line. They respond to red light (the right-hand peak centred around 600 nm) and also to blue light (the lesser peak centred around 450 nm) So when ‘spectral violet’ hits our eyes, it tickles our S-ones (blue) and our L-Cones (red). We perceive purple.
Answer:
The Violet in ROYGBIV, the mnemonic many people use to remember the colors in a rainbow, is a misnomer, says Henry Reich of Minute Physics in the video above. The reason we say violet is because Isaac Newton said violet, but when Isaac Newton said violet he really meant blue.