# The Naked Scientists Forum

### Author Topic: Why are there only 7 colours in a rainbow?  (Read 13567 times)

#### omid

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##### Why are there only 7 colours in a rainbow?
« on: 15/09/2009 17:33:15 »
why there are only 7 colors in a rainbow can some one please clear????????????
« Last Edit: 23/09/2009 03:39:07 by chris »

#### neilep

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##### Re: Why are there only 7 colours in a rainbow?
« Reply #1 on: 15/09/2009 18:27:46 »
...Actually there's millions..even billions and zillions of colours as they gradually fade from one to the other.

lookee here

Quote

(PhD physicist responds) The eye perceives six colors in the rainbow. Red, orange, yellow, green, blue and violet. (some people include the violet color of Indigo as a separate color.) But there are also colors in the infrared and ultraviolet present as well. The rainbow is caused by diffuse refraction of sunlight in water droplets. The source of the light is the sun's photosphere. While each atom in the photosphere may emit light at one quantum frequency, the sun is so hot that doppler shifting of the light causes the lines to "fuzz out" so that you see essentially a continuous spectrum of light. The strictly correct answer, would then be, "an infinite number"

« Last Edit: 15/09/2009 18:30:48 by neilep »

#### SkepticSam

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##### Re: Why are there only 7 colours in a rainbow?
« Reply #2 on: 15/09/2009 18:45:53 »
A rainbow doesn't only have 7 colours, it's just that you can not tell the subtle differences between them. The colours range from those in the ultra violet through those that the human eye can see and on to the infra red.

What is interesting is that each colour we see is from a single rain drop. The red is from high altitude drops down to the drops at lower altitude that produce the Violet. This means that there are many millions of drops to each rainbow.

It's the size of those rain drops that dictate the brightness of the rainbow. If the drops are greater than 1000 micrometers then the blue is a dull colour but the red, yellow, and orange are bright. If they are smaller than 300 micrometres then it is reversed. the redder colours are dull / weaker and the bluer ones are brighter. Drop sizes between 300 and 1000 micrometers generally have even brightness of colours.

Should you have drops below 30 micrometers in size, then the colour of the bow will appear very feint or even White. Whilst this is not seen with rainbows it can be seen in fogbows.

http://www.atoptics.co.uk/rainbows/primcol.htm [nofollow]

#### Bored chemist

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##### Re: Why are there only 7 colours in a rainbow?
« Reply #3 on: 15/09/2009 19:27:09 »
Newton said there were 7 because it was a lucky number.

#### John Chapman

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##### Re: Why are there only 7 colours in a rainbow?
« Reply #4 on: 18/09/2009 09:33:21 »
And why doesn't a rainbow include brown (tea colour) or white (the colour of sugar. Two please Omid, stirred anti clockwise is the way I like it).

#### omid

• Neilep Level Member
• Posts: 1016
##### Re: Why are there only 7 colours in a rainbow?
« Reply #5 on: 18/09/2009 11:00:52 »
And why doesn't a rainbow include brown (tea colour) or white (the colour of sugar. Two please Omid, stirred anti clockwise is the way I like it).

Hey John!
A rainbow doesn't include brown and white color thats why NO tea for you

#### John Chapman

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• Posts: 602
##### Why are there only 7 colours in a rainbow?
« Reply #6 on: 24/09/2009 09:29:48 »
Before my Grandad died he was an artist and worked in oils. He used to paint prolifically, although not so much today. He would look at an object and, instead of simply saying “that's green“ he would say that it was a combination of several different greens plus a bit of a particular blue plus a touch of yellow. He would mix it all up and it would be spot on. So, was the finished paint reflecting each of the constituent colours separately, which somehow averaged into the correct green or was there now a single wavelength which was the new colour?

Computer applications, such as Photoshop, use a colour selector which is typically a single square from which supposedly any colour can be selected. Now presumably my monitor makes every colour using just three primary colours and pixel brightness, so why do our brains see a single colour instead of a mixture of three? And how does the primary colour model fit in with the colours of the rainbow? And am I allowed to start a sentence with the word ‘And’? These are the essential questions, I think.

#### Karen W.

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##### Why are there only 7 colours in a rainbow?
« Reply #7 on: 24/09/2009 10:09:54 »
It is amazing information here...

A rainbow doesn't only have 7 colours, it's just that you can not tell the subtle differences between them. The colours range from those in the ultra violet through those that the human eye can see and on to the infra red.

What is interesting is that each colour we see is from a single rain drop. The red is from high altitude drops down to the drops at lower altitude that produce the Violet. This means that there are many millions of drops to each rainbow.

It's the size of those rain drops that dictate the brightness of the rainbow. If the drops are greater than 1000 micrometers then the blue is a dull colour but the red, yellow, and orange are bright. If they are smaller than 300 micrometres then it is reversed. the redder colours are dull / weaker and the bluer ones are brighter. Drop sizes between 300 and 1000 micrometers generally have even brightness of colours.

Should you have drops below 30 micrometers in size, then the colour of the bow will appear very feint or even White. Whilst this is not seen with rainbows it can be seen in fogbows.

http://www.atoptics.co.uk/rainbows/primcol.htm

I had no idea that the size of droplets dictated the intenseness of color... that is very cool as I often wondered why some rainbows were so pale in color as to others that were so much more vibrant! Thanks SkepticSam!

#### Karen W.

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##### Why are there only 7 colours in a rainbow?
« Reply #8 on: 24/09/2009 10:11:14 »
why there are only 7 colors in a rainbow can some one please clear????????????

omid That is a good question.. I am glad you asked it!
I did  not know either!

#### Karen W.

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##### Why are there only 7 colours in a rainbow?
« Reply #9 on: 24/09/2009 10:19:30 »
...Actually there's millions..even billions and zillions of colours as they gradually fade from one to the other.

lookee here

Quote

(PhD physicist responds) The eye perceives six colors in the rainbow. Red, orange, yellow, green, blue and violet. (some people include the violet color of Indigo as a separate color.) But there are also colors in the infrared and ultraviolet present as well. The rainbow is caused by diffuse refraction of sunlight in water droplets. The source of the light is the sun's photosphere. While each atom in the photosphere may emit light at one quantum frequency, the sun is so hot that doppler shifting of the light causes the lines to "fuzz out" so that you see essentially a continuous spectrum of light. The strictly correct answer, would then be, "an infinite number"

That is amazing to think about.. when I think of all the colores that I can imagine It boggles my mind.. its like when I was a kid a special gift we were given one Christmas were these two huge 4 tiered carousel caddy's of Crayola crayons with colors that were unimaginable and I remember feeling like a kid in a candy shop and was oh so excited about the variety that I was literally taken aback..... boy..... I loved drawing and art and color was my favorite I really do not like neutral rooms I want intense bright colors intertwined in my environment they rock....Rainbows are incredible aren't they!

#### Bored chemist

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##### Why are there only 7 colours in a rainbow?
« Reply #10 on: 24/09/2009 19:28:21 »
And why doesn't a rainbow include brown (tea colour) or white (the colour of sugar. Two please Omid, stirred anti clockwise is the way I like it).

It does include brown and white if it's a rainbow trout.

#### The Naked Scientists Forum

##### Why are there only 7 colours in a rainbow?
« Reply #10 on: 24/09/2009 19:28:21 »