# Why does light diffract into only seven colours?

• B
louis_slicka
Summary:: If relativity causes wavelengths to change I.e the redshift effect (or blue shift effect for objects moving towards us) does the presence of 7 different colours denote seven different changes?

If the doppler effect shows that direction of trajectory changes wavelength and colour of light does that mean the seven colours of the rainbow are a breakdown of seven different universal direction changes or that an object with a preposterous gravitational mass has entered and left our universe seven time's? Causing the seven different splits in colours of light?

davenn, russ_watters, weirdoguy and 1 other person

## Answers and Replies

Mentor
Summary:: If relativity causes wavelengths to change I.e the redshift effect (or blue shift effect for objects moving towards us) does the presence of 7 different colours denote seven different changes?

If the doppler effect shows that direction of trajectory changes wavelength and colour of light does that mean the seven colours of the rainbow are a breakdown of seven different universal direction changes or that an object with a preposterous gravitational mass has entered and left our universe seven time's? Causing the seven different splits in colours of light?
Welcome to PF.

The naming of some of the colors is for human convenience; there is actually a continuum of frequencies in the visible spectrum, as with the rest of the EM spectrum:

https://en.wikipedia.org/wiki/Light

sysprog, davenn and russ_watters
Homework Helper
But why seven??It is a good Talmudic number and we seem to like it as humans. I really have no idea why we have seven. Do the Inuit describe seven basic colors? When was the spectrum first so divided ?
After all we have only three distinct eye pigments and yet I swear there are seven colors when I look at the rainbow.
Roy G Biv, your rainbow is a mystery to me.

sysprog and russ_watters
Mentor
Abhishek11235, sysprog, russ_watters and 3 others
Homework Helper
So you have binary vision.
Dangit I see seven colors in the CIE chart too...

sysprog and berkeman
Gold Member
2021 Award
... does that mean the seven colours of the rainbow are a breakdown of seven different universal direction changes ... ?
No, it means you don't understand the rainbow spectrum. See Berkeman's post directly above.

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sysprog
Gold Member
2021 Award
One should be aware of the fact that "color" is a physiological rather than a physical phenomenon. It's much more complicated to describe than just the frequencies/wave lenghs in a spectrum of the electromagnetic wave field.

The rainbow is a continuous spectrum, i.e., you have a continuum of frequencies/wave lengths in the solar spectrum (which is pretty much thermal radiation). I also don't see a discrete set of "seven colors" but a continuum in a rainbow.

sysprog, Pyter, vela and 3 others
Homework Helper
I also don't see a discrete set of "seven colors" but a continuum in a rainbow.
It may well be a sign of age and familiarity but my brain certainly wants to break up that continuum into seven sections. Is there any psycho-linguistic research...I think I will give it look-see.

Edit: This is pretty interesting about cultures and rainbow colors:

https://theconversation.com/red-yel...w-the-worlds-languages-name-the-rainbow-68641

sysprog, vanhees71 and PeroK
Homework Helper
Gold Member
2021 Award
I'm not sure we haven't been trolled here. "... an object with a preposterous gravitational mass has entered and left our universe seven times?" Really!

Delta2 and sysprog
Mentor
I'm not sure we haven't been trolled here. "... an object with a preposterous gravitational mass has entered and left our universe seven times?" Really!
Hah, that's funny. I never finished reading the OP before replying. We'll give him one more day to reply to this thread and go from there...

sysprog and hutchphd
Homework Helper
I'm not sure we haven't been trolled here.
and I got interested in something tangential and wandered off into the woods...

sysprog and vanhees71
Mentor
I see 4096
Obviously you haven't gotten the video card upgrade to 16.7 million colors...

Abhishek11235, sysprog, davenn and 4 others
Gold Member
2021 Award
my brain certainly wants to break up that continuum into seven sections
Sure, and if you look carefully you'll see that all 7 of them have shades of color, not just one color.

sysprog and vanhees71
Gold Member
The human central nervous system (of which the retina is one part) can normally only produce sensations of three primary colors. That limits the amount of nameable colors we can describe. Some people, all of who are women, can sense four primary colors because they have four types of retinal cone cells with different pigments. The ability for this is coded in the X-chromosome, that's why male people can't have that ability. Some species of birds can even sense five primary colors. As far as I know, the US military is preliminarily researching a possibility to make people see infrared radiation with some kind of eye drops that temporarily add new pigment molecules to your retina. That way you could see in the dark and locate people by "seeing" how their body temperature is higher than ambient.

https://en.wikipedia.org/wiki/Tetrachromacy

sysprog, Pyter, Hamiltonian and 2 others
Mentor
sysprog and davenn
Gold Member
does the presence of 7 different colours denote seven different changes?
No, it represents 7 different sizes of T-shirts: XS, S, M, L, XL, XXL, XXXL. Now if you ask me why in Universe there are no T-shirts of other sizes, it's due to quantization, obviously. Duh!

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phinds, sysprog, Pyter and 5 others
Staff Emeritus
2021 Award
There are also seven sizes of olives: medium, large, extra large, jumbo, giant, colossal and super colossal.

Coincidence?

Anyway, I am late for my next universe.

sysprog, vanhees71, Demystifier and 4 others
Mentor
FWIW - one language I'm familiar with has different names for colors - green turquoise when translated to Navajo is either blue stone or sometimes 'sky' stone because it the same color as the clear sky. In Navajo.
This is not a unique feature of just one language.

Our brains use the language we speak and it's color designations. Plus we get fooled:
Adelson - move your cursor off the line drawn box completely and back onto the checkerboard a couple of times...

hilbert2
Staff Emeritus
Homework Helper
russ_watters, DaveC426913, Merlin3189 and 4 others
Staff Emeritus
From archeology, not physics, comes the argument that ancient humans did not see the color blue. Should the OP modify his argument to make it 6 colors, not 7?

https://thedoctorweighsin.com/history-purple-blue/

Pyter and hutchphd
James Demers
Fun fact: not all colors are found in the rainbow. There's no magenta, and no cyan. Which proves that "color" isn't just a function of wavelength - it's the end result of our visual and neural systems' processes. (Any yellow you think you see on your computer screen isn't really there ... at least, it's not due to yellow light, which the screen cannot produce.)
Makes you wonder what other colors might be "seen" by different species with different wetware.

Mondayman, vanhees71 and Pyter
Homework Helper
Gold Member
And I think George Miller might suggest that we generally perceive seven gradations of anything.

darth boozer
"Some people, all of who are women, can sense four primary colors..."

So why does my wife only see red when I suggest going for a beer?

When looking at a spectrum, at what point does "red" become "orange" etc.? What we call colours are a man-made concept and because of the way we perceive colour, with three frequency-dependent structures and an amplitude-dependent structure, we see six basic colours plus black and white as well as every shade in between.

russ_watters and davenn
Gold Member
I read Newton shoehorned in extra colors so they'd match up with the seven notes in the musical scale. See, for example, https://sciencetrends.com/7-colors-rainbow-order/.
This is my recollection too. Newton decided there should be seven. (I think he inserted indigo )

nasu, russ_watters and hutchphd
Gold Member
But why seven??It is a good Talmudic number and we seem to like it as humans. I really have no idea why we have seven. Do the Inuit describe seven basic colors? When was the spectrum first so divided ?
After all we have only three distinct eye pigments and yet I swear there are seven colors when I look at the rainbow.
Roy G Biv, your rainbow is a mystery to me.
People see 3 or at most 4 colors in a rainbow. The whole 7 color thing originates with Newton and his mysticism.

hutchphd, vanhees71 and DaveC426913
rsk
But why seven??It is a good Talmudic number and we seem to like it as humans. I really have no idea why we have seven. Do the Inuit describe seven basic colors? When was the spectrum first so divided ?
After all we have only three distinct eye pigments and yet I swear there are seven colors when I look at the rainbow.
Roy G Biv, your rainbow is a mystery to me.
I read that Newton could actually only see 6 colours and invented a 7th to fit his number fetish/superstitions.

I always tell this to my students when they protest that they can't tell the difference between indigo and blue/violet.

hutchphd
artis
I only see "50 shades of grey"... oh wait, it's because I'm watching an old Hitchcock movie.
object with a preposterous gravitational mass has entered and left our universe seven time's?
I find your label of gravity "repulsive"..., I would call it rather "size + gravitational mass" , I mean we don't want to offend any gravitational mass out there, after all gravity is all about "attraction"...

Gold Member
I read that Newton could actually only see 6 colours and invented a 7th to fit his number fetish/superstitions.

I always tell this to my students when they protest that they can't tell the difference between indigo and blue/violet.
Newton never went as far as constructing the CIE Chromaticity chart which identifies a range of non-spectral colours. There a colours round behind white and nowhere near the spectral locus. So it has to be seven plus.

Staff Emeritus
2021 Award
Please, everybody knows there are 96 colors. (Used to be only 64)

Mondayman, vanhees71, rsk and 3 others
Homework Helper
and only the rich kids got to see in 64 colors..

That way you could see in the dark and locate people by "seeing" how their body temperature is higher than ambient.

Here's one news article about that, but it's about research conducted by the Chinese:

https://www.slashgear.com/night-vision-eyedrops-nanoparticles-research-infrared-vision-28567893/
I think the distinction between "night vision" (which is Near Infrared (NIR) often actively illuminated usually silicon detected devices) and thermal imaging FIR devices needs to be emphasized here. Only the thermal imaging devices will see the glowing enemy combatant in the dark field but these are very different from usual silicon optics or eyeballs.
I once had to explain this to a client who had already spent a fair sum of money trying to develop a silicon CCD-based imaging thermometer. Lack of understanding can be expensive, and I felt like the Grinch as I invoiced him for my services.

vanhees71
LCSphysicist
LCSphysicist
What was that i had just read

LCSphysicist
Please, everybody knows there are 96 colors. (Used to be only 64)

View attachment 289324
Mine was just twelve