A rainbow is formed from a continuous spectrum of wavelengths of light, so why do we perceive it as seven separate bands of colour?
Matt Chamings Barnstaple, Devon, UK
Our perception of a rainbow is coloured (pun intended) by our expectation that there are seven colours. In reality, there aren’t seven distinct bands, but multiple colours blending and shading into one another.
Robert Senior Uppingham, Rutland, UK
Around 1665, Isaac Newton performed experiments with a prism producing a spectrum in which he identified seven colours.
Prior to that, the spectrum had been thought to have five colours. For example, in his book Experiments and Considerations Touching Colours, Robert Boyle described the spectrum he produced with a prism as “denoting the five consecutions of colours Red, Yellow, Green, Blew, and Purple”.
This book was published in 1664, just before Newton started his experiments. However, the number seven had long been considered mystical, denoting perfection and completeness. This sort of mysticism fascinated Newton as much as science, so he thought there must be seven colours in the rainbow. He added orange and split purple into indigo and violet.
I am a volunteer at Woolsthorpe Manor, Newton’s family home, and have often demonstrated his prism experiment to visitors. I have never been able to see indigo and violet as separate colours.
Mike Follows Sutton Coldfield, West Midlands, UK
It is said that Isaac Newton only perceived five colours in the rainbow and added two more (orange and indigo) because the number seven had mystical significance. We have seven days of the week, seven natural notes in most Western music and, in Newton’s time, only seven planets had been discovered.
While this was simply one of Newton’s mystical beliefs – others included alchemy and the philosopher’s stone, which could turn base metals into gold – it has become part of our cultural inheritance.
In the UK, we are taught the mnemonic “Richard of York gave battle in vain”, so we look for the corresponding colours in the rainbow. Perhaps another culture would have come up with a different number of colours and a different mnemonic.
Luce Gilmore Cambridge, UK
The retina has cones with three opsin pigments that are sensitive to different colour bands, peaking in orangish-red, green and deep blue. However, in the retina, colour information is encoded into red-versus-green and yellow-versus-blue channels for transmission to the optic cortex. Hence the four basic colours of visual psychology are red, yellow, green and blue.
Cyan looks distinct from blue because both green and blue cones are firing in that band, so it appears brighter. Similarly, orange is towards the yellowish part of the red band where green cones are still firing strongly. Owing to the retinal encoding, the band we perceive as yellow is very narrow, whereas the red band is very broad. Also, violet appears reddish due to that encoding.
Keith Ross Villembits, France
We only have three colour receptors in our retinas. These respond to red, green and blue light. If two of these receptors are activated at the same time, we see three more colours: yellow (red + green), cyan (green + blue) and magenta (blue + red). These are also used in colour printers, and the last one isn’t a rainbow colour. If all three receptors are activated, we “see” white. Our computer and TV screens use the same primary colours as our eyes.
People often get confused by being told at school that the three primary colours are yellow, red and blue. Actually, these artistic colours are, from a physics perspective, the secondary colours of yellow, cyan (an artist’s blue) and magenta (an artist’s red).
The difference is that your eyes and a TV screen start with no light and you gradually build up to white, whereas your printer starts with white paper and so, as you add ink, you start subtracting colours from the white.
To answer this question – or ask a new one – email email@example.com.
Questions should be scientific enquiries about everyday phenomena, and both questions and answers should be concise. We reserve the right to edit items for clarity and style. Please include a postal address, daytime telephone number and email address.
New Scientist Ltd retains total editorial control over the published content and reserves all rights to reuse question and answer material that has been submitted by readers in any medium or in any format.