The color white

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Is it supposed to be a frequency of light or a combination of all colors or both? What does it mean when a prism splits up a color?

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Color is the interpretation by our brain of the light our eyes detect. It is not an inherent property of light.

If you look yellow on a computer screen you are, in reality, you eyes get red and green lights but because how your eyes work is interpreted it the result if just yellow light detects it.

There are spectral colors that are a color we can experience our eye is hit buy a single wavelength of light, The color of the rainbow or a prims is spectral colors.

There is also extra-spectral color, that is colors that are not spectral colors, so you need multiple wavelengths of light or just no light. White is an extra-spectral color. It is light with a wavelength distribution just like the light from the sun on the surface of the earth. It will be made up of all spectral colors of light. The amount of light depends on the wavelength.

It is not just white that is an extra-spectral color; gray, black, brown, magenta, pink, etc is not something you can see if a single wavelength of light hit your eye.

White light is made up of many different frequencies of light. Some other colours are also made up of a mix of frequencies – magenta, for example. Others (yellow, for example) might be, or might not be.

When light enters glass at an angle, it changes direction. That’s because the speed of light in glass is slower than in air.

Different frequencies of visible light have slightly different speeds in glass, so their direction changes differently – that’s how a prism “splits” light into different frequencies – the higher frequency “blue” component is bent more than the green or red, so a thin beam of white light gets split into a rainbow of colours.

Colours are how we *perceive* the different mixtures of frequencies of light, but our colour sense is pretty limited. Light has a full continuum of possible frequencies, but our eyes lump them into three bands we perceive as “red”, “green” and “blue”, which each stimulate diffferent cells in our retina.

Some light stimulates just the “red” cells, and will look red. A slightly higher frequency will stimulate red AND green cells, and we see that as yellow. Higher still, and the red cells aren’t stimulated any more, and the light looks green.

So it’s possible to have “yellow light” which is a single, pure frequency, and a different “yellow light” that’s actually a mix of green and red, that looks exactly the same because it stimulates the red and green cells exactly the same way as the yellow light – though a colourblind person might see a difference.

There are also “blue cells” in our retina that respond to even higher frequencies of light.

If we stimulate all three types of cells equally, all at once, we perceive that as “white”, but there’s no single frequency of light that does that. Sunlight has a pretty even mix of all frequencies, so it seems white to us. A computer or phone screen will generally just mix three specific frequencies (red, green and blue) to produce white.

Colour is complicated.

Light is any kind of EM radiation; *visible* light is EM radiation in the part of the spectrum that our eyes respond to.

Every photon has a wavelength; any tiny patch of light can have a vast mix of photons at different wavelengths and intensities.

Think of a violin; there’s no frets on it, so you can play not just any note, but anywhere between notes, as loud or quiet as you want.

Now imagine you can have a million strings on your violin, playing any conceivable monstrous chord, each string at its own pitch and its own volume.

And that’s one million-string violin *per pixel* (or per retinal cell).

There’s just too god damn much information. There’s no way to make a cell that can report all the incoming wavelengths and their exact relative proportions – and even if you could, you’d need a brain the size of an office building to process it all. This is big data on a scale that’s just plain *stoopid*.

So, we cheat by massively simplifying the whole thing down to just four numbers per pixel.

Take four different kinds of retinal cells per tiny patch of retina, three with a response-curve that peaks at a different part of the visible light spectrum, and one that measures overall brightness.

From the relative strength of each cell’s response in each quad, you can reconstruct a three-string approximation of each insane monster chord. It’s not the same thing, not at all, but there’s a consistent translation between the two – which is all we really need.

Something’s only hitting the long-wavelength response? We call that red.

And by comparing that response with the total brightness, we can guess at the mix of frequencies. If the total brightness and low-frequency response are similar, then the overall mix is up around the red-peak in the spectrum, and that’s your bright fire-truck red.

If we’re only seeing a little red response but a bunch of total brightness, then it must be mostly far down the low end, because it’s hitting the tail of the curve – and that’s your deep burgundy territory. If it were hitting the high-end tail of the red response, the mid-frequency (green) cells would be responding too (and that’s where you start getting into oranges and yellows).

Rinse and repeat for the other two peaks, and you can infer a whole bunch about the original mix of wavelengths.

What we call ‘white’ is when *all three* peaks are getting hit equally. Usually this means a broad sweep of frequencies right across the spectrum, a huge blaring _**HNORT**_ of every single string from eg: the giant thermonuclear fireball we call the sun.

However, that doesn’t *have to* be the case: if you peer really close at a white part of your screen (you may need a magnifying glass or a water droplet), you’ll see that it’s actually just red, green and blue light mixed together equally – just three single strings being played in the first place, one wire-skinny little *ploink*.

Our eyes use the same three-note chord to represent *both* cases; we literally can’t tell the difference between them.

And your screen replicates all the other colours the same way. Just three colours out can generate the same visual response as virtually any spread-spectrum smear of wavelengths you want, because we bought our colour perception from Wish.

Except orange. Screens *suck* at producing a believable orange, because maths. There’s just one place where the formula for replicating colours falls down badly, and it’s right there. Don’t believe me, image search the brightest, most vibrant orange you can find anywhere, and hold an actual orange up next to it. It sucks.