Why are there only seven colours in a rainbow?

Answer 1

There aren't only seven (the people above are wrong)
every single colour exists in a rainbow going from red to violet (and probably a few that the eye can't see like infra-red and untra-violet). It looks like there are only seven because our eyesight isn't good enough to see the colours blending into eachother.

Answer 2

there aren't. It's just a convention. By convention we decide to call "colour" a certain range of wavelengths of light. Fine, but of course the rainbow contains a broad spectrum of frequencies.

Many cheap colour pencil boxes will have 20 colours, and high-end ones will have 100 or more colours.

You could say that a rainbow has 100 colours, or 1'000 colours. A trained eye can probably fairly well identify at least 50 colors / nuances.

Seven just happens to be a popular number for many things, but 10 is also popular and would have done the trick also.

Answer 3

I agree with most of the answers already given. There is an infinite range of hues including those at either end of the spectrum that are invisible to the human eye. I think the reason we have limited the named colours to seven is for reasons of simplicity but also because of language.
We can all identify with the words red, orange, yellow etc. and if asked to pick out samples that fit those labels we would probably pick out something quite recognisably red or yellow or whatever. (Unless we are colourblind, of course, which leads off into a whole other discussion about rods and cones...)
The difficulty comes when we start to use more specific terms for colours - scarlet, crimson, magenta. Look at the names on the tubes of paint in an art shop for an indication of the range of man-made colours available where there has been an attempt at conformity. To get even more precise descriptive terms numbers are used. Look at the descriptions of colours used on a computer or Pantone colours. To add even more to the confusion manufacturers of domestic paints or dyes like to use terms that are evocative of a mood. Have a look in the local DIY store to see what I mean.
So no wonder we only list seven rainbow colours - we wouldn't be able to use the mnemonic "Richard Of York Gave Battle In Vain" if there were more!

Answer 4

So I agree with the stuff above, there are three primary colours of light and mix these to make the seven, but don't forget that there is actually a continuous spectrum of wavelengths of light that we artificially split seven ways out of convention.

However, not all "colours" are in the rainbow - that is a common misconception. This is because some colours are made up by mixing short and long wavelengths. Brown, for example is a typical mix. The nature of a rainbow is that it splits out the wavelengths and separates them. So you don't get these mixes in a rainbow, but you do see them in nature.

Answer 5

Because after all we are only human, we have learned, decided and evolved to only see seven colours, but if you get out some paint and try a challenge, then see how many colours, shades and tones you could make from seven colours.

Aren't rainbows beautiful anyway a magic thing in our lovely world.

Oh what a hippie answer that was ;-). Mind you have you ever tried actually seeing all seven colours in a rainbow? It can be quite difficult and they all merge together.

Answer 6

The seven colours you can see in a rainbow are the colours of the spectrum. Red, Orange, Yellow, Green, Blue, Indigo and Violet can also be seen when sun light hits a prism.

There are 3 primary colours, Red Yellow and Blue. From these three colours you can make the three Secondary colours by mixing two primary colours:
Red /Yellow = Orange
Red/ Blue = Purple
Yellow/ Blue = Green

Answer 7

There are infrinite colours in the rainbow - we have just named seven as it's easier! All of the other colours are mixes of the other ones.

Answer 8

they are the main colours of the spectrum but there are variations between them and after all they are only as described by humans and to a dog for example they might only see it as 2 colours. SO a creature with superior sight may ask 'why are there only 1billion and 2 colours in a rainbow?' Was that at all helpful, I think not!

Answer 9

There aren't. There are as many colours in the spectrum (or rainbow, if you like) as there are wavelengths of visible light. The colours of the spectrum were named by Sir Isaac Newton, who was not only one of the greatest scientists of all time, he was a bit of a loony. He believed that numbers held mystical properties and that seven was imbued with the mysteries of the universe and what have you. Nobody has ever bothered correcting him.

Answer 10

There arent only seven colours in the rainbow!!!
The conventional colours are seven, but what about the interludes, i.e., how the colour changes from violet to indigo to blue.....!
There are all the possible combination of colours, from the wavelength size (from Violet to Red), in the rainbow.

Answer 11

Actually, there are only 6 - indigo is just a merge of blue and violet.....
But anyway, it's the colours light splits into when you put it through a prism. Light is white, because it's made up of loads of different colours: if you made a spinner and coloured in a section in each colour of the rainbow, then spun it, it would look white - all the colours merge together to make one; but when you put it through a prism, it's separated out into those 6 colours.