Why does the rainbow have only 7 colours?
Two reasons. Firstly it's worth
considering that these "colors" aren't distinct colors, they are just
names we give to parts of the rainbow that we consider to be distinct,
as you've mentioned, it's a continuous spectrum, we've just added in
distinct names to certain ranges of the specktrum for both biological
reasons and socialogical reasons. Read on:
Biological: The human eye has three colour receptors, Red, Green, and Blue, each of them respond to a small section of the spectrum, and they overlap. Forget that they are colours, just consider them simple receptors that respond to different signals.
If we receive a signal that causes the Red receptor to report a high reading but not any other color, we know this as the color Red. For example Wavelength at say 650nm in the above chart we would see as Red.
If we receive a signal that causes the Green receptor to report a high reading, we know this as the color Green.
If however, we receive a signal that causes BOTH the Red and Green receptor to report a high reading, we know this as Yellow. Note there are TWO ways we can see Yellow, if we see a single wavelength of about 570nm, OR if we seen two distinct wavelengths at about 530nm and 650nm (and that's the key of how our computer and TV screens work).
By this definition, there should only be five distinct colors in the rainbow corresponding to how our color receptors respond to it:
- Red
- a mix of Red + Green (Yellow)
- Green
- a mix of Green + Blue (Cyan)
- Blue
And of course variations where there's a little more of one than the other.
Interestingly, we also see a sixth color when our Red and Blue receptors both report a high reading. Red + Blue, we call this Magenta, but that color doesn't actually exist in the rainbow - there isn't a single wavelength of light that can cause our Red and Blue receptors to respond at once, it has to be a mix of at least two different wavelengths for us to experience this color, for example 460nm and 650nm.
So you can see how our RGB computer and TV screens can trick our eyes into seeing many many colors - putting a red light (650nm) and a green light (530nm) on at the same time causes our eyes to report EXACTLY the same signal as if there was a single yellow light (570nm) there instead!.
This graph also gives a hint at why certain colours like Yellow are more noticeable than Teal, our Green and Red receptor response overlap quite a lot, giving us very good ability to see Yellow, as it causes a large response in both red and green receptors. But the diagram is a bit misleading, a more accurate one would be:
You can see that sensitivity of Blue is lower, and Teal/Cyan/Turquoise kind of occupies that dip there, making our eyes less sensitive to it.
But this is only shows that the rainbow should have a minimum of 5 easily-discernable colors. We're taught that the rainbow has 7!
Social: why does the rainbow have 7 colors? Because we're taught that it has 7 colors. From a young age, we are taught to recognise different colors: VIBGYOR. But take a look at violet and indigo. What's the actual difference? Ok, so indigo is a bit more blue than violet, but most people would have a hard time deciding between the two if seen in isolation. I'd personally just call both of them "purple", like many do.
Can you REALLY see discrete indigo and violet bands in this rainbow?
The fact is, we think the rainbow has 7 colors because we are taught it has 7 colors. Why can't it have 9 colors? Violet, Indigo, Blue, Cyan, Green, Lime, Yellow, Orange, Red? Why wasn't Cyan and Lime in the original lineup? They're nice colors, and you wouldn't have a problem recognising them. Or would you?
As it turns out, yes, some people do have problems, and I'm not talking people with colorblindness. It turns out that our ability to discern colors has a lot to do with practice, and language. BBC's Horizon program covered some great research into how the Himba tribe in Africa have a remakable ability to discern between to very similar (to us) shades of green, but have difficulty telling the difference between other colors that we don't have a problem with. It was a program called "Do you see what I see?". I couldn't find a non-removed youtube clip, but there is one here: Colour is in the eye of the beholder
I can say that I've noticed that many of my grandparents had real difficulties discerning green and blue, I'm not sure why this is, but perhaps it's a similar reason as the above.
So the answer to why are there "7 colors in the rainbow?" It's because that's what we teach our kids, because teaching them more color names like "coral, "olive", "lime", "teal", etc. would be too difficult (and those are just the colors in the rainbow, there are more that aren't in the rainbow, like the various lighter/darker shades, variations on magenta like "maroon", etc.). So really we should be saying: "The rainbow has countless colors, but we like to group them into 7, because unless you become a designer, that's all you'll ever know or use."
Biological: The human eye has three colour receptors, Red, Green, and Blue, each of them respond to a small section of the spectrum, and they overlap. Forget that they are colours, just consider them simple receptors that respond to different signals.
If we receive a signal that causes the Red receptor to report a high reading but not any other color, we know this as the color Red. For example Wavelength at say 650nm in the above chart we would see as Red.
If we receive a signal that causes the Green receptor to report a high reading, we know this as the color Green.
If however, we receive a signal that causes BOTH the Red and Green receptor to report a high reading, we know this as Yellow. Note there are TWO ways we can see Yellow, if we see a single wavelength of about 570nm, OR if we seen two distinct wavelengths at about 530nm and 650nm (and that's the key of how our computer and TV screens work).
By this definition, there should only be five distinct colors in the rainbow corresponding to how our color receptors respond to it:
- Red
- a mix of Red + Green (Yellow)
- Green
- a mix of Green + Blue (Cyan)
- Blue
And of course variations where there's a little more of one than the other.
Interestingly, we also see a sixth color when our Red and Blue receptors both report a high reading. Red + Blue, we call this Magenta, but that color doesn't actually exist in the rainbow - there isn't a single wavelength of light that can cause our Red and Blue receptors to respond at once, it has to be a mix of at least two different wavelengths for us to experience this color, for example 460nm and 650nm.
So you can see how our RGB computer and TV screens can trick our eyes into seeing many many colors - putting a red light (650nm) and a green light (530nm) on at the same time causes our eyes to report EXACTLY the same signal as if there was a single yellow light (570nm) there instead!.
This graph also gives a hint at why certain colours like Yellow are more noticeable than Teal, our Green and Red receptor response overlap quite a lot, giving us very good ability to see Yellow, as it causes a large response in both red and green receptors. But the diagram is a bit misleading, a more accurate one would be:
You can see that sensitivity of Blue is lower, and Teal/Cyan/Turquoise kind of occupies that dip there, making our eyes less sensitive to it.
But this is only shows that the rainbow should have a minimum of 5 easily-discernable colors. We're taught that the rainbow has 7!
Social: why does the rainbow have 7 colors? Because we're taught that it has 7 colors. From a young age, we are taught to recognise different colors: VIBGYOR. But take a look at violet and indigo. What's the actual difference? Ok, so indigo is a bit more blue than violet, but most people would have a hard time deciding between the two if seen in isolation. I'd personally just call both of them "purple", like many do.
Can you REALLY see discrete indigo and violet bands in this rainbow?
The fact is, we think the rainbow has 7 colors because we are taught it has 7 colors. Why can't it have 9 colors? Violet, Indigo, Blue, Cyan, Green, Lime, Yellow, Orange, Red? Why wasn't Cyan and Lime in the original lineup? They're nice colors, and you wouldn't have a problem recognising them. Or would you?
As it turns out, yes, some people do have problems, and I'm not talking people with colorblindness. It turns out that our ability to discern colors has a lot to do with practice, and language. BBC's Horizon program covered some great research into how the Himba tribe in Africa have a remakable ability to discern between to very similar (to us) shades of green, but have difficulty telling the difference between other colors that we don't have a problem with. It was a program called "Do you see what I see?". I couldn't find a non-removed youtube clip, but there is one here: Colour is in the eye of the beholder
I can say that I've noticed that many of my grandparents had real difficulties discerning green and blue, I'm not sure why this is, but perhaps it's a similar reason as the above.
So the answer to why are there "7 colors in the rainbow?" It's because that's what we teach our kids, because teaching them more color names like "coral, "olive", "lime", "teal", etc. would be too difficult (and those are just the colors in the rainbow, there are more that aren't in the rainbow, like the various lighter/darker shades, variations on magenta like "maroon", etc.). So really we should be saying: "The rainbow has countless colors, but we like to group them into 7, because unless you become a designer, that's all you'll ever know or use."
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